Each object in VHDL has to be of some type, which defines possible values and operations that can be performed on this object (and other objects of the same type). The set of operations of a type consists of:

explicitly declared subprograms that have a parameter or result of the particular type; such subprograms can be either predefined (in standard packages) or user-defined;
basic operations (assignments, allocators, selected names, indexed names, slice names)
numeric literals, literal null, string literal, bit string literals, aggregates or predefined attributes - depending on particular type.
There are four classes of types in VHDL:
scalar types (values of these types have no elements),
composite types (values of these types consist of element values),
access types (provide access to objects of a given type) and
files (provide access to objects that contain a sequence of values of a given type).

See respective topics for details.

Apart from predefined types (available through the packages Standard and Std_Logic_1164), the user can define his/her own types. A user-defined type can be of any of the four classes mentioned above. The rules for defining types are described in detail in the corresponding topics.

Important Notes

A type defines not only a set of values, but also a set of operators.
VHDL is strongly typed language which causes that two types defined in exactly the same way (i.e. lexically identical) but differing only by names will be considered different.
If a translation from one type to another is required, then type conversion must be applied, even if the two types are very similar (like assigning a natural variable to an integer variable).