basically there are two types of lift kits... body lifts and suspension lifts.
Suspension lifts essentially put more separation between the axels and the frame and body lifts put more separate between the body and the frame...
Body lifts are generally useless outside of providing more clearance between the tires and the body. From certain angles they can make the truck look bigger but IMO they look funny if that's all you've done. Like a normal height person with really short legs and an enormously tall torso.
Suspension lifts will put more separation between the axels and the fram and give you more clearance everywhere but the axels... from a profile view it gives the truck a much taller look. My truck (94 Toyota) is actually shorter then most full-sized pickups but looks taller simply because it has a whole lot more air under it.
Suspension lifts are also more expensive. Body lifts can be stupid cheap depending on your vehicle. I have a few friends that just bought a sleeve of hockey pucks, drilled some holes and volia... 2" of body lift for about $25. If you're not installing it yourself you'll probably end up paying MORE for a body lift simply because it involves removing the bed/cab to insert the lift between the two. Suspension lifts you can often install just putting the truck up on jacks. Though it's still pretty involved.
Usually the hardest part of lifting a vehicle isn't the lift itself but all the other parts that are effected. if the radiator is mounted to the body instead of the frame you'll have to remount it, you stearing column might need to be extended, some hoses, lines and wires might need to be extended. You might need to remount your rear differential to keep the U-joints from breaking under the added stress. You might need to install some new equipment up front to correct the stearing geometry... etc. etc. etc. Each truck is different.
If you do go for it IMO you should go for a suspension lift before a body lift, and only go with a body lift to help compliment an existing suspension lift if you need more tire clearance.
Also keep in mind that you'll want to get bigger tires to compliment the lift (usually looks pretty stupid without them, I'd recomend BFG all-terrains for on-road and mud-terrains for off-road), and if you get bigger tires you might also need to install bigger fender flares (get bushwackers).
Tires will run you another $600 or $700 and fender flares will run about $300 or $400 (if left as unpainted black plastic)
Lots to think about... but having a lifted truck is a whole lot of fun. Intimidating other cars and just being able to drive pretty much wherever you damn well please.