Solidity Programming Language – Data Types

What is the difference between signed and unsigned integers?

The key difference between signed and unsigned integers is that signed integers can store both positive and negative values, while unsigned integers can store only non-negative values. 

Bytes:

 In Solidity, the term “bytes” refers to a dynamically-sized byte array. Solidity provides two types of byte arrays: fixed-size arrays (called “bytesN”, where N is a number between 1 and 32) and dynamic arrays (simply called “bytes”).