Is there any difference between "all X", "all of the X", "all the X"? E.g.,
- all friends
- all of the friends
- all the friends
All friends is global: it means all friends of anybody anywhere, everybody who may be characterized as a friend. "All friends are to be cherished."
All the friends (or All of the friends—there is no difference of meaning, as the responses to the question cited above by Xavier Vidal Hernández tell you) means all of a specific group of friends—the friends who roomed together at Princeton in 1983 or the current Friends of the Art Museum. "All the friends met for dinner on Tuesday."
This is complicated a little by the fact that in a context where a group of friends has previously been specified, explicitly or implicitly, the definite article may be omitted. For instance, if a notice in the Newsletter of the Friends of the Art Museum states "All friends are invited to dinner on Tuesday", this is unlikely to be misinterpreted.