Ornamentation is the general term for the techniques associated with dressing up tunes. These techniques include the roll, tip, cut, cran and triplet. A player/singer won't play a tune in the same way from verse to verse or from performance to performance. Variation, either rhythmic, melodic, phrase or harmonic, is another way of dressing up tunes. How different performers use ornamentation and variation depends on his/her style.
A cut is like a grace note, i.e. a quick extra note played above the main note.
A tip is the opposite of a cut. i.e. the extra grace note is lower.
A roll is a combination of cuts and tips. Rolls can be short or long.
A triplet involves playing 3 notes in the time of 1 beat.
The ornaments above are fairly universal. That is, they are used on all melody instruments. Some ornaments are associated with particular instruments but it is common now for techniques associated with a particular instrument to be played on other instruments. (see "changes" notes)
The Cran is an ornament associated with piping. On a low d, its not possible to play a roll as there is no lower note. Therefore the piper uses several notes above the main note to simulate a roll. a, g, and f# are alternated with the main note d.
Double stopping involves playing 2 notes together, the bottom one usually being an open string and is obviously associated with the fiddle, perhaps in imitation of the drone of the pipes. This technique is often borrowed by accordion and concertina players.
The slide is associated with the fiddle where the player slides between notes, especially on slow airs. Dont mix this up with the slide dance which is in fact a fast single jig.
Variation is a priciple where performers vary tunes every time they play them. Rhythmic variation involves changing the rhythm. Melodic or intervallic variation involves changing the pitch of a tune. Phrase varation is where the player changes the phrasing. e.g. breathing in different places or using different bowing. Harmonic variation is where the change (melodic really) actually changes the implied harmony.
Again it is important to stress that its up to the player to uses these techniques as they see fit. A good player will improvise them. A really good player will have an element of composition about them. i.e. there is a sence of direction, a structure to the whole performance.