Ska is basically an early strand of Jamaican music developing from the late 50's onwards. It is the forerunner of much of the reggae that followed and was at it's peak (imho) in the mid-late sixties. my own favourite exponents are the Skatellites.
Ska rules! I would recommend getting some Trojan samplers to listen to which will let you hear some great tunes from loads of different people. If you can afford it I would even get the Trojan ska boxset (they also do reggae). It wasn't that expensive if I remember rightly. If you like the Madness stuff get some 2 tone compilations which will have a good selection and give you an idea of what was about. The good thing about the compilations is that you can find out who you like before delving deeper.
There's also Punk Ska like a lot of No Doubt's tracks. As well as the Punk Ska covers of things like Come On Eileen and the Fraggle Rock themetune, which are both excellent!!
Just a thought......i've recently got an album by a London group called King Prawn, who are very much modern ska, with a bit of Red Hot Chili Peppers and Rage ATM thrown in.....you might wanna give that a go? It's brilliant!
Ska has three main waves. There is the original ska, pioneered by artists like Derrick Morgan and Prince Buster, two-tone (like Madness, The Specials, etc) and ska-core, otherwise known as 3rd wave, popularised by people like No Doubt, Save Ferris, Spunge, etc, which is more punky. Hope that helps!