Runequest has no classes per se. Your hero can do everything he wants. Characters have cultural backgrounds and professions, which gives them skills on certain skills, but in game character can use and advance in any skill he wants. So, It is possible to develop a character wearing heavy plate armour and being skillful with mighty greatsword, also being experienced sorcerer.
Take a look for MrqWiki, there is MRQ SDR Luxury Edition (Mongoose Runequest basic rules) available for download.
http://mrqwiki.com/wiki/index.php?title=Main_Page#Downloads
SRD doesn't have all things found on books, but you should get in to the system with them - and even play.
Comparing to D&D, here is some opinions:
* In Runequest, characters are free to do anything they want and learn any skills they want - no class restrictions.
* System is LOT simplier than D&D. Also lot more flexible, you can easily create own rules or systems without breaking the system.
* skills are percentiles - for example, your Axe skill might be 60, which means when you try to hit, you roll d100 dice, and result 60 or under is successful. There might be modifiers in question also (for exampe, trying to hit enemy which is prone will give bonus)
* Character hit points never increase (well, actually they will increase a little bit, but not so much). This means that even hero-level character only has quite few hitpoint, and even one good blow can be serious. Also, characters have separate hit points in different body locations.
* Deadly battles (because of low hit points)- you need strategy, and running away or surrender is really sometimes needed. Even hero level characters will die easily
