Aid can be a verb or a noun, and in both cases it refers to the same thing: help.

1. Aid as a noun means help or assistance. There are a number of more specific ways that it can be used though.
Ex- After the tornado, the country required international aid.

2. As a verb, aid means "to help or assist." You can aid someone else by giving them food when they are hungry, for example, or just by helping them in any way.
Ex - I will apply for financial aid so that I can go to school this year.

3. Aide is a noun and it refers to someone who works as an assistant to someone else, usually an important person.
Ex- The president has too many things to do everyday, so he has his aides take care of some things.