The difference is in the wrapper. Egg roll wrappers have egg in them and are thicker. Spring roll wrappers were originally made in the springtime and the wrappers are thinner and crisper when fried. The insides can be anything.

From what I've found the definition is up to the place making them. I used to think that it was that spring rolls are vegetarian, but I have found some places that put meat in spring rolls. I'll be following this question to see if there is a solid definition. Great question!

the lack of egg in the spring roll