I'm not going to flame you OP, not everyone knows these things and sometimes the best thing to do is just ask. Since this topic is so general, and no offense, very simple. It probably can't be found by a search on this forum, although a search on google would probably do the trick.
What the above posters have said pretty much sums up the boost questions and how it is increased.
My contribution I wanted to add is that in the most general form, bolt on simply increase the efficiency of the engine. Increasing the efficiency will in turn increase the horsepower. For example a DCI increases the efficiency in which air can enter the engine and DPs do the same for air exiting the engine.
A tune will do more intricate things that you can learn in tuning threads on here. The Internet is a plethora of information on this, just read, it really is pretty fascinating how it works.
I have exhaust and an intake...