The Difference Between a Spring Roll and an Egg Roll
Let's start with flaky and crispy versus crunchy and chewy.
Your experience of an egg roll or a spring roll begins with an important visual distinction. In fact, this may be the largest difference between the two. An egg roll's surface is covered in small bubbles and a spring roll's surface is perfectly smooth. And then there's the matter of the bite—at first chomp, an egg roll crunches and is a little chewy while the outside of most spring rolls will shatter into several thin flecks of light and crispy dough as soon as your teeth crack the surface.
The dough for egg rolls is made (as the name suggests) of flour and eggs, giving it a sturdier texture and making it more like fried pasta. The dough is also rolled to be a little thicker than a spring roll, and after it's filled and wrapped, the entire roll is deep fried. The origin of the egg roll travels no further east than New York City and no further back than the early 20th century. Even still, it's become a definitive staple of American Chinese cuisine. The spring roll, however, has a much more complex and longstanding history in Asia.
Spring roll dough contains no egg, making it lighter and crispier than the egg roll when fried. And since variations of the dish have been made across thousands of miles in Asia over the span of centuries, there are numerous fillings and methods used to create the ever-diverse spring roll. Many sold in America are still fried in a wok and have a lightly golden exterior, but be prepared to find some spring rolls with uncooked and see-through shells that are packed with cold fresh vegetables in the sushi section at your grocery store.
As far as difference in fillings, egg rolls are typically filled with cooked cabbage, some other vegetables, and pork, while spring rolls are often simply filled with cooked vegetables. But like any generalization for a food so beloved worldwide, you're sure to find exotic exceptions if you look hard enough.