You can only have two classes active at any time but, except for racial restrictions, you can actually have all of them to choose from. Visiting a House Maid will allow you to change your currently-active Primary and Secondary classes.
Remember that you can use a 
Home-sweet-home rune to portal to your house from anywhere. There, just talk to the House Maid by your front door to change your Primary and Secondary classes.
Combining More Than One Class
This is possibly the single thing that sets Runes of Magic apart from all other MMORPGs. In RoM you can have more than one class at a time, and actually have the ability to learn all (or most) of them.
Primary Class
Your Primary is your main class. All skills of that class will be available to you, plus any non-exclusive skills of your Secondary class, and certain Elite Skills available only to those with the specific combination you are using.
Secondary Class
This controls what additional skills you may have, in addition to the skills of your Primary class.
For instance, although a Warden cannot heal himself except through certain weapon skills, if you are a Warden/Priest or a Warden/Druid, you will be able to use some basic Buff and Healing Spells belonging to your Secondary class.
Tertiary Classes
Although you can only have two active classes at a time, having an extra class, or five, to choose from makes you a more well-rounded and universally useful player.
To get your third class you can talk to Johnny Dutin [Eye of Wisdom] in the Hall of the Eye of Wisdom in Varanas. You must be at least level 20 in both of your first two classes to do this. He starts the quest, [10] The Price for Improvement: A Promise.
Additional classes may be unlocked by purchasing (for 699
($20.86)† from the Item Shop) and using a 
Class Maximum Expansion Ticket.
What is the Most Useful Class?
Please see this post on the Joint Forums for a good guide to the most useful classes, by role.
Ultimately, the right class combo for you is the one you enjoy playing!
The Classes
| Class | Combat role | Resource | Recommended armor | Recommended weapon |
|---|---|---|---|---|
 Warrior |
Melee DPS | Rage | Chain (and shield) | Swords or axes |
 Scout |
Ranged DPS (physical) | Focus | Leather | Dagger or sword, and bow or crossbow |
 Rogue |
Melee DPS | Energy | Leather | Daggers, poisons, and projectiles |
 Mage |
Ranged DPS (magical) | Mana | Cloth | Staff or wand |
 Knight |
Tank | Mana | Plate and shield | One-handed hammer |
 Priest |
Healer | Mana | Cloth (and shield) | Staff or wand |
 Warden |
Tank or melee DPS | Mana | Chain | Two-handed axe or two-handed sword |
 Druid |
Healer or ranged DPS (magical) | Mana and nature's power | Cloth (and Shield) | Staff or wand |
 Champion |
Melee DPS or tank | Rage | Chain | Hammers |
 Warlock |
Ranged DPS (magical) or Support | Focus and soul points (psi) | Cloth | Staff or wand |