Oracle treats them grouped as follows in its documentation:

• CHAR : fixed length

• VARCHAR2 and VARCHAR : variable length

• NCHAR and NVARCHAR2 : Unicode data

CHAR

The data type CHARstores fixed-length character strings . When you create a table with one column CHAR, you must specify a string length (in bytes or characters) between 1 and 2000 bytes for the width of the columnCHAR . The default value is 1 byte.

Oracle warrants that:

• When you insert or update a row in the table, the value for the CHAR column has the fixed length .

• If you give a shorter value, then the value is padded blank to the fixed length.

• If a value is too large, Oracle Database returns an error.

The Oracle database compares the values ​​of CHARusing the blank-padded comparison semantics .

VARCHAR2

The data type VARCHAR2 stores variable-length character strings . When you create a table with a column VARCHAR2, you specify a maximum string length (in bytes or characters) between 1 and 4000 bytes for the column VARCHAR2. For each row, Oracle Database stores each value in the column as a variable-length field unless a value exceeds the maximum column length, in which case Oracle Database returns an error. The use of VARCHAR2and VARCHARsaves space in the table.

For example, suppose you declare a column VARCHAR2with a maximum size of 50 characters. In a single-byte character set, if only 10 characters are given for the column value VARCHAR2in a particular row, the row's column stores only the 10 characters (10 bytes) and not 50 .

Oracle Database compares the values ​​of VARCHAR2using unsupported comparison semantics .

VARCHAR

The data type VARCHARis synonymous with the VARCHAR2 data type. To avoid possible changes in behavior, always use the data type VARCHAR2for storing variable-length character strings .

Regarding the semantics of length for character data types

Globalization support allows the use of multiple character sets for character data types. Globalization support allows you to process single-byte and multibyte character data and convert between character sets. Client sessions can use client character sets that are different from the database character set.

Consider character size when specifying column length for character data types. You should be aware of this issue when estimating space for tables with columns that contain character data.

The length semantics of character data types can be measured in bytes or characters.

Byte semantics treat strings as a sequence of bytes. This is the default value for character data types.

Character semantics treat strings as a sequence of characters. A character is technically a codepoint from the database character set.

For single-byte character sets, the columns defined in character semantics are basically the same as those defined in byte semantics. Character semantics are useful for defining variable - width multibyte strings ; Reduces complexity by defining the actual length requirements for data storage.

For example, in a Unicode (UTF8) database, you must define a column VARCHAR2that can store up to five Chinese characters along with five English characters. In byte semantics, this would require (5 * 3 bytes) + (1 * 5 bytes) = 20 bytes; In character semantics, the column would need 10 characters.

VARCHAR2 (20 BYTE) y SUBSTRB (<cadena>, 1, 20)they use byte semantics.

VARCHAR2 (10 CHAR) y SUBSTR (<cadena>, 1, 10)they use character semantics.

The parameter NLS_LENGTH_SEMANTICSdecides whether a new character data type column uses byte or character semantics. The default semantic length is byte. If all character data type columns in a database use byte semantics (or all use character semantics), users do not have to worry about which columns use which semantics. The qualifiers BYTEand CHARshown above should be avoided where possible, as they lead to mixed semantic databases . Instead, the initialization parameter NLS_LENGTH_SEMANTICSmust be set correctly in the server parameter file ( SPFILE) or in the initialization parameter file, and the columns must use default semantics.

NCHAR and NVARCHAR2

NCHARand NVARCHAR2 are Unicode data types that store Unicode character data . The character set of data types NCHARand NVARCHAR2can only be AL16UTF16or UTF8and is specified at database creation time as the national character set. AL16UTF16and UTF8they are Unicode encoding.

The data type NCHARstores fixed-length character strings that correspond to the national character set.

The data type NVARCHAR2stores variable-length character strings.

When you create a table with a column NCHARor NVARCHAR2, the specified maximum size is always in character-length semantics. Character-length semantics are the default, single-length semantics for NCHAR or NVARCHAR2.

For example, if the national character set is UTF8, the following statement defines the maximum byte length of 90 bytes:

CREATE TABLE tab1 (col1 NCHAR (30));

This statement creates a column with a maximum character length of 30. The maximum byte length is the multiple of the maximum character length and the maximum number of bytes in each character.

The maximum length of a column NCHARis 2000 bytes. It can contain up to 2000 characters. Actual data is subject to the maximum byte limit of 2000. The two size constraints must be satisfied simultaneously at run time.

NVARCHAR2

The maximum length of a column NVARCHAR2is 4000 bytes. It can contain up to 4000 characters. Actual data is subject to the maximum byte limit of 4000. The two size constraints must be satisfied simultaneously at run time.

See also: Oracle Database Globalization Support Guide for more information on data types NCHARandNVARCHAR2

Using Unicode data in Oracle Database

Unicode is an effort to have a unified encoding of every character in every language known to man. It also provides a way to represent privately defined characters. A database column that stores Unicode can store text written in any language.

Oracle Database users who implement globalized applications have a strong need to store Unicode data in Oracle Databases. They require a data type that is guaranteed to be Unicode regardless of the database character set.

Oracle Database supports a reliable Unicode data type through NCHAR, NVARCHAR2and NCLOB. These data types are guaranteed for Unicode encoding and always use character-length semantics. The character sets used by NCHAR/ NVARCHAR2can be UTF8or AL16UTF16, depending on the national character set setting when the database is created. These data types allow Unicode character data to be stored in a database that may or may not use Unicode as its database character set.

Implicit type conversion

In addition to all the implicit conversions for CHAR/ VARCHAR2, the Oracle Database also supports implicit conversion for NCHAR/ NVARCHAR2. Implicit conversion between CHAR/ VARCHAR2and NCHAR/ is also supported NVARCHAR2.

• Source: Oracle Documentation

In a very concrete way.

nvarchar: Te puede almacenar cuaquier tipo de datos UniCode.
varchar: La columna tiene una restriccion de 8 bits.
nvarchar2: La columna tiene un limite mayor a 4000 bits, y debes especificar 

the maximum length you want. As far as I know, Nvarchar2 is only used in Oracle. One of the features allows you a greater number of special characters.

char is a variable length data type. This is of ANSI data type. Values ​​shorter than the column size are not well padded with the column size. Is worth to say

Dato a guardar          Dato guardado        
char(5) = '';             _____
char(5) = 'a';            ____a 
char(5) = 'asdas';        asdas

( "_" be considered as a space).

In this case, those 5 spaces are always saved.

nvarchar This in turn limits the maximum size of a VARCHAR to 8,000 bytes. NVARCHAR means "almost" the same, with the difference that you can store unicode data, that's where the N (uNicode) comes from.

varchar2 and varchar stores variable-length character strings determined by the (required). Its range is from 1 to 4000 characters. Is worth to say:

Dato a guardar              Dato Guardado
varchar(5) = '';            
varchar(5) = 'a';           a
varchar(5) = 'asdas';       asdas
varchar(5) = 'asdasa'; // Generará Error

I hope this is clearer for you.

NOTE:

Consider that the different database engines have certain variations, but in essence it works this way.

At the moment, varchar and varchar2 are synonyms. VARCHAR2 is reserved by Oracle to support distinction between NULL and empty string, without being totally sure version 12c would already be included. VARCHAR does not distinguish between a NULL and an empty string, and never will.

If it depends on empty string and NULL is the same thing, VARCHAR2 should be used.