The main distinction between in and into is that there is implied movement with into. You cannot be "into" a place, but you can move into it. In colloquial English, "in" can be substituted in many cases. You could say "I went in the room," although "I went into the room" would be more accurate.
(English no longer has the concept of direction and motion that used be present in the language: here, hence, and hither meant "in this place," "from this place," and "to this place," respectively; likewise there/thence/thither and where/whence/whither. Some other languages maintain this distinction: e.g., in Russian, the use of где / куда / откуда is mandatory, and Swedish distinguishes between var and vart. But hence, whither, etc. has disappeared from Modern English, except in certain set phrases.)