Every character has a character level and class levels. The character level is the sum of any and all class levels and race levels that character may have. A characters class level is the number of levels he may have taken in any one class plus half the total number of levels rounded down in any other class he has taken. For example: Bregalad is an Elf Druid 2/Warrior 3/Barbarian 3. Since his race (elf, all basic races have 0 racial levels) has no racial levels and he has 2 levels in Druid, 3 levels in Warrior, and 3 levels in Barbarian he is an 8th level character. Now, when calculating his class level for the purposes of determining the effectiveness of his abilities and powers, his Druid class level is 5 since he has 2 levels of Druid and 6 levels of other classes (2 + 6/2 = 2 + 3 = 5). His Warrior class level is also 5 since (3 + 5/2 = 3 + 2.5 = 5.5) which gets rounded down to 5.
There is no limit to the number of classes one character can take, but it is usually best to limit this number. The best way to determine a characters base attack bonus, defense bonus, and saves is through fractional addition.