The advantages and disadvantages of the different vocations:
Knights are the masters of close combat fighting. They advance faster than any other vocation in their fighting-skills. Knights are rather average at distance fighting, but they only have very basic magic spells. Knights often train their skills. This is the most popular vocation in Tibia, and probably the one that costs the least gold.
Paladins[6b] are best at non-magical distance fighting. They advance in distance fighting faster than the other vocations, but only average in close combat skills. Paladins are able to use quite a number of useful magic and produce a few helpful runes. They constantly have to buy new ammunition for their ranged weapons, but they can train with spears.
Sorcerers[6b] are masters of aggressive magic. They can learn many rune-spells and aggressive spontaneous spells, but they also know some defensive magic. Their skills (close combat as well as distance fighting) generally are very low, they seldom advance.
At the beginning a sorcerer-character is difficult and expensive to play, but the higher you advance the stronger you become. Probably the least popular vocation.
Playing a magical vocation costs a lot and involves standing around eating food and waiting for the mana to go up for a long time. You will spend much of your Tibian life killing rotworms and similar monsters in close, non-magical combat to earn the money for magical attacks on stronger monsters.
Druids[6b] can learn many defensive, but also some aggressive spells, and are also able to produce (and sell) healing runes. They can convice monsters with their runes. Very much like the sorcerers, the druids seldom advance in the non-magical skills. Druids, too, grow much stronger with each level they gain.