The & operator does string concatenation, that is, forces operands to be converted to strings (like calling CStr on them first). +, in its turn, forces addition if one of the expressions is numeric. For example:
1 & 2
gives you 12, whereas
1 + 2
"1" + 2
1 + "2"
give you 3.
So, it is recommended to use & for string concatenation since it eliminates ambiguity.