There are a lot of different kinds of extensions. I personally like to go with what's called a 'tape' in extension. (No, I thought the same thing at first, we aren't talking like scotch tape!) It is a tape-like glue that isn't hot. You stick two tabs together with a small section of your hair in the middle. I like to use these extensions because I feel they are less damaging to the hair. I have clients that think their hair doesn't grow (which it does, they get roots) but they use these extensions and their hair grows by leaps and bounds.
There are some clients that have weak hair from (whether it be from medications or diet) and these are not the ideal clients for extensions. The weight of the extensions overtime can pull off their hair because it's not strong enough to hold the extensions. If you don't fall into that, these extensions are amazing. I like to use them for clients looking for length, thickness, color or even all three. I also have many brides have me put them in for all wedding activities. (Many turn into "lifers" after) With these extensions you can wash and dry as normal (just takes a little longer, remember it's double the hair your used to). The cost of the hair is expensive, but you have to remember the hair is lasting you for six months so your buying hair twice a year if you take good care of the extensions. They do need to be moved up, back to your root, every 6-8 weeks with your stylist.
Other types of extensions are hot glue in beads. I am not a fan of these. They easily can create knots in your hair, ripping out your natural hair. I also don't like the idea of hot glue being melted to the hair. These beads of hair come in smaller clumps of hair. The way it lays can look more unnatural depending on the look your going for.
There are also sew in extensions where the natural hair is braided and the hair is sewn into the braid.
Clip-in extensions are another option. These are good for short term use or filler for an updo. These can be tricky to be placed right in your hair and to be styled correctly to blend with your natural hair. You do have to be careful and make sure your clip-in extensions are real hair. It's very common for them not to be and once you put heat to them (curling iron, wand or flat iron) they will melt if not real.