There are lots of types of hair extensions. The newest are "keratin" they are glued to your hair and if you take care of them they can last 5,6 months. They are done professionally and the price varies but it can cost up to 10$ per strand of hair so about 700$ for your whole head (it depends). They will tell you in the salon how to wash it, if you go to the beach or pool tie it into a braid and it wont get ruined. You should ask around which salon is the best because if it isn't done professionally it call fall out after you wash it the first time and it can ruin your hair also it should be taken off professionally after five moths. There are different types of extensions there are also clip in extensions(you probably heard of them) you can clip them in yourself and take them out when you want to. You have them at better accessory stores.
Here are some helpful videos.
Keratin (the ones I was talking about)
http://www.youtube.com/watch?v=COpQgTbmBis
Clip in
http://www.youtube.com/watch?v=iRXMTjCgG1k