what is an integer?

Answer 1

Similar to what Bebida said. I'd just add:

An integer is a whole number, that can be positive or negative. e.g. -6076 430 0 are all integers, but 6.7 is not.

An unsigned integer is non-negative, i.e. 0 and above.

Answer 2

It depends on the machine, but mostly, a 4 byte number. A number that can be expressed using 32 bits.