They mean the same thing, but "some of" requires a pronoun, a determiner or an article to follow it. Examples: Some of the people who work here are from an employment agency. Some of his friends are vegetarians. Some of these books are good, but most of them are boring. In all cases, "some of" refers to a subset/part of a group of things. "Some" refers to a larger, less precise group of things. Examples: Some people can be content playing bingo and paying rent. Some insects bite people, but most don't. Some movies are in 3-D, but not too many yet.