Green Kryptonite, although rare, is the most common type. It is harmful to natives of the planet Krypton. If one is exposed long enough, they could go into a Kryptonian trance or it will turn their skin green from Kryptonite poisoning, and not long after it could kill them. Typically, by the time the skin is green, it's too late. Villains use Green K as a weapon to defeat Superman. Green K was a key element in Doctor Natas's Noxium crystal.
Although Red K has no fatal effects on Kryptonians, it's effects are still unpredictable. It could give Superman extra arms, or it could even turn him into a child. It could do a ton of other things. Red K is unpredictable, there's no telling what may happen. But one thing about these unpredictable effects, is that they are only temporary, usually only lasting about a day.
Blue K isn't harmful to Kryptonians at all, in fact, when injured, it can heal them. However, it's effects on Bizarros (such as Bizarro himself) is deadly. It can be created by using the faulty duplicator that produced Bizarro on Green Kryptonite, although naturally-existing pieces have appeared in a few instances.
Krypton Steel is another form of Kryptonite, that is also presumably a very rare form. It's basically Krypton's version of the earthmetal known as "steel." It's harmless to Kryptonians, but it's also indestructible to anything except for Superman, or possibly another equally powerful being. Some forms of Krypton steel however, are even indestructible to Superman.
↑(Although this seems unlikely, since Solomon Grundy and Bizarro are both roughly the same strength as Superman, and yet they didn't even attempt to open a box made of Krypton steel. Instead the Legion tricked Superman into doing it for them)