O-Castitatis-Lilium

I am a little hesitant about downloading anything when it comes to Minecraft, because...well I'm not the most tech savvy person. Though tekkit really is interesting looking from game footage and stuff like that, I was wondering...how doe sit work? like, what do you have to do when you install it and, do I have to host a server or others or something...I'm not too sure how to word it well...umm...can I just open up my minecraft and then go to single player and just play or is there something different. I'm just really confused as to how it works is all. if someone could give me a bit of a description or whatnot, or even direct me to where I can read about it a little more, that would be awesome! Thanks in advance for the info and help!