A type is a formal designation of what kind of data a given item is. For instance, it might be a number, or a character. Types usually include more specific details; e.g. for numbers, they might be signed or unsigned, floating point or integer, and there is usually an indication of the precision (i.e. number of bits).
The use of structures or similar mechanisms may allow a programming language to support even more complex types; e.g. the 'BIGNUM' subroutine package of LISP allows the use of arbitrary precision integers.