TTAF (Total Time AirFrame) is how much flying time the aircraft has done - this is important as most aircraft have a designed lifespan - when it is up major components may no longer have sufficient strength to be safe, at which time the aicraft can either be donated to a museum (and most museums that want Cessnas have enough already) or sell it for its weight in scrap aluminium. A really low time aircraft is less likely to have cracks and corrosion that would need tending to, meaning periodic maintenance would be cheaper. In most cases (unless this is really high) it is unlikely that the typical private pilot would use up all the remaining time. Despite this a high TTAF on a Cessna may mean it was used for training, which means a greater likelihood of abuse, damage, previous repairs and future headaches. Much like buying a high mileage car whose history you don't know. This all said though, you will pay significantly more for a low-time aircraft so you will pay for it either way, regardless.

SMOH is time Since Major Overhaul (also seen as TTSMOH) - because of the critical nature of anything on an aircraft, all major components on an aircraft need to be taken apart and inspected by an approved mechanic on a periodic basis (determined by the particular component's history of failure, in hours of operation), and the more recently this has been done, the more use you'll get out of the aircraft before you'll need to make this major expenditure (which can run $5-20,000 and up without any difficulty).
Normally the biggest item is the engine, and typically it must be disassembled, any worn items either reground or replaced and then reassembled and tested. Hourly rates to do the work are significantly higher than for auto mechanics, and the prices on replacement parts are also a lot higher. Don't expect any bargains on labour unless you get the training to do it yourself, and parts only if you use uncertified parts which cannot be legally used on a certified aircraft such as a Cessna (or an airliner).
The worst part is you will need to do this regardless of how well it is running.
Corrosion is also a common issue, especially if the aircraft is in a damp climate or is stored outdoors in which case it must be dealt with, and despite the common misconception, aluminum does corrode - or at least the alloys used in aircraft construction (as do the steel fittings that hold everything together).

Thanks Nieuport, that was valuable info i wish to own an aircraft in future and your info will be valuable in purchasing one.