Tease and denial, for me, is about ramping up the sexual energy of the bottom. Teasing them into a state of frustration, desperation and intense need for release, and then taking the stimulation away. This can put some people into an intensely submissive (and sometimes even worshipful) mind state, which is the thing about engaging in it as a dominant that pleases me the most. It can go on for a long time. When I have negotiated a lover's orgasms under my control, I like to tease them a lot to remind them that only I have the power to make them come. I have ended my T&D, short-term and extended, in a few ways: an orgasm, a ruined orgasm (those are harder to control), walking away, and a chastity cage. The cage turns T&D into straight up denial. MmMmm.
Edging is a really delicate process of getting to the brink of orgasm and then giving yourself a tiny break to cool down, then getting yourself to the brink of orgasm (which, as you go on, becomes more and more intense) and then cooling down, over and over again.... you're in a constant state of near-orgasmic bliss. Finally, when/if you DO come, the orgasm tends to be totally mindblowing. I edge myself all the time. If you fuck up the timing, though, this can easily lead to a ruined orgasm. It can also be longer term (think days or weeks). This state of near-orgasm is said to be good for creativity over longer periods of time. I don't know first-hand because I'm greedy and like to come a lot.
I can't really speak to the merits of ruined orgasms. They're not something I'm into giving or receiving, 'cause they make me sad. I'm sure someone else reading this who enjoys them can give you some insight.