The simple difference for questions is: "may" is for asking permission; "can" is asking if something is possible. By "possible" you could also mean "allowable", which is why you'll often hear "can" used in place of "may". But use "may" to accent the politeness of the request. We sometimes tease people who use "can" in place of "may". :D "Can I make a phone call?" "Why are you asking me? Only you know if you can operate a telephone or not!"

Although, traditionally, can has meant “to be able” and may has meant “to be permitted” or to express possibility, both can and may are commonly used interchangeably in respect to permission. For example; May/Can I go to the mall tonight? Regardless of whether you choose can or may here, it is clear that permission is being requested. In spoken English, a request for permission is generally answered with can, cannot, or can’t, rather than with may or may not, even if the question was formed using may.

This is actually a common mistake that some people make all the time so you are not alone, just remember the meaning of the word "can" and "may". the word "can" as a verb means to be able to, (ex. He can read very well.), to know how to, (He can play soccer.). the word "may" as a verb is used to express possibility (ex. It may rain today.), it can also be used to express permission (ex. May I come in?)

Los dos significan la posibilidad de hacer algo, en definitiva 'poder'. El 'may' sin embargo, conlleva la idea de permiso, requiriéndose autorización de la parte a la cual tu pides algo. Si no te quedo muy claro, en esta página sale muy clara la explicación: http://es.wikibooks.org/wiki/Ingl%C3%A9s/Verbos_defectivos