a statement about what will happen in the future

(What is the part of speech of 'what'? Is 'what' an interrogative pronoun or a relative pronoun?)

In this (not a complete sentence yet!), "what" is an interrogative pronoun. It's introducing an indirect question.

Thank you.

1. a statement about what will happen in the future

2. a statement about the things which will happen in the future

(What about this one? Is #1 the same as #2?)

Both mean the same thing, except that 1 has that indirect question in it introduced by "what," while 2 has a relative clause introduced by "which."

http://speakspeak.com/resources/english-grammar-rules/questions/structures-of-indirect-questions

http://www.chompchomp.com/terms/relativeclause.htm

The part of speech of 'what' depends on its function in a sentence. In the given statement, 'what' is used as a relative pronoun. Relative pronouns introduce relative clauses, which provide additional information about a noun in the main clause. In this case, 'what' is introducing the relative clause 'what will happen in the future', which modifies the noun 'statement'. Therefore, 'what' is functioning as a relative pronoun here.