This information can be found in the first few pages of the book. Steinbeck takes great care to develop each of their characters so that they are strikingly different, but very able to be friends.
George is short with chiseled features whereas Lennie is tall, big and somewhat less featured. This may be on purpose to illustrate a smarter and not so smart character. Lennie has a mental disability.
Both love each other but express it differently. George can be rather harsh on Lennie in terms of tone of voice, while Lennie can pour on a guilt trip like no other. George presents himself as angry, Lennie looks for sympathy. Both are happy to work toward their goal of having a house together one day. As the book continues, readers get the idea that Lennie believes in this more than George and George may use the idea to illicit good behaviors.
I like to describe George as if he were a dad, and Lennie a kid. Steinbeck paints animal images of Lennie by having his bear paw grab water to drink, and his capacity to think and get excited compares to a dog.
Lennie has superior strength and works as if he were two men.