What's difference between 'few' and 'a few'?

Few have negative and A few has positive meaning.

Example : I have few friends
 Practically it means no friends.

I have a few friends.
Practically it means a small number of friends


More example :
a few means :- small number . used for countable things such as boys,people.

e.g: There were only a few students in the class.

Few means :- no e.g There were few students in the class so the class could not start

So, it easy :)