Much of this will be familiar territory to anyone who has played T&T, although some of the details will wind up differing...
Warriors – The fighters of Trollslayers, who obviously excel at combat compared to the other classes. They have the easiest time wearing armor, as they are the most practiced with it. To become a Warrior requires a minimum score of 9 with the three physical attributes, STR, DEX, and END. Ideally, these attributes should be higher, especially as different weapons have various minimum scores for STR and DEX.
Magicians – Call them wizards, witches, sorcerers, what have you, these are the men and women who know how to lay down the mojo and smack you around with it. A minimum of 13 in both KNO and PRE is required, as well as a minimum DEX of 9. They are not forbidden from wearing armor, but take extra DEX penalties from doing so, due to lack of experience. Also, they must make sure that wearing heavy armor does not drop their DEX below the minimum of 9. As spellcasting usually requires two free hands, they rarely employ shields. They are not forbidden from using various weapons, but as spellcasting can drain STR, they tend toward lighter weapons as a result.
Experts – This covers a wide variety of Professions, which mainly cover anyone who is not considered a Warrior or a Magician. Experts who are adventurers often include such professions as Woodsmen, Priests, Bards, and Thieves, which provide useful skills for an adventuring lifestyle. There are no attribute requirement for Experts. Some experts can learn how to cast magic spells (this requires the same attribute minimums as Magicians have), but they generally have to spend twice the amount of time and money to learn spells that a Magician would; the exception being if the spell ties in thematically to the Expert's profession (i.e. a Woodsman learning plant- or animal-based spells, a Bard learning music- or charm-based magic, etc.). They can wear any sort of armor, handling it better than Magicians but not as well as Warriors. They can use any weapon they have the minimum STR and DEX for. They tend to have more Talents to call upon than the other classes.
Paragons – A rare breed, Paragons are skilled in both the ways of the Warrior and the Magician. This requires a minimum of 13 in all primary attributes. While they can cast spells while wearing armor, the fact that spellcasting can drain from either PRE (which affects Health) and/or STR (which affects their ability to use weapons) means that they might not choose to do so while directly in combat. Also, they must make sure that wearing heavy armor does not drop their DEX below 9, the minimum needed for spellcasting. Likewise, the use of shields in combat will usually preclude spellcasting while doing so. Finally, because they have concentrated their studies into two opposing fields, they have little time left over for any other hobbies or interests, and as a result have fewer Talents than any of the other classes. (Yes, I need a better name for this class...)