The are mainly three common variations of control knobs, if you categorize them through shapes, Bell, Speed, and Domed.
Bell Knobs
Bell Knobs, also called UFO or Hat knobs are commonly found on Strats and some Les Pauls. On Strats, they are usually a solid color with embossed numbers, on Les Pauls, they are usually clear with the numbers and the color painted on the inside.
Speed Knobs
Speed Knobs are chunkier and many guitarists find them easier to use than Bell Knobs. They are about the same width as a bell knob but have no taper. They are usually constructed from plastic and have the numbers and color painted on the inside.
Domed Knobs
And finally, we have Domed Knobs (some of them could be flat top domed knobs). They are commonly found on Telecasters and a number of Super Strats. Domed Knobs are usually made of metal or plastic and have a knarled edge making them easier to grip with sweaty fingers.
When it comes to knobs that fit different potentiometer shafts, there are also 2 different types. We'll talk more about pot shaft types in another article.
Pots come in two basic styles, Split-shaft and Solid-shaft. Keeping things simple, Push on knobs go on Split-shaft pots, Screw on knobs (those that have a small screw in the side of the control knob) go on Solid-shaft Pots.
Push-on Knobs
However, it isn't quite as simple as that. Depending on where your pot was manufactured it will either have 24 splines (US CTS Pots) or 18 splines (Import pots). Control knobs that fit a 24-spline pot shaft will not perfectly fit on an 18-spline version, and vice versa. However, there are universal control knobs available that will fit on both.
Screw-on Knobs
Some Strat control knobs and most Dome knobs are screw mounted and will therefore need to be attached to a Solid-shaft pot. However, they can be attached to a Split-shift pot if you screw into the side of one of the shafts, not where they meet.