You can charge whenever circumstances permit a charge. To charge a character must move in a straight line, and must move at least 10'. This is significant because if an enemy with equal or more speed than yourself closes with you, you generally have the choice between taking no more than a 5' step (subjecting yourself to a full melee attack) or moving more than 5', subjecting yourself to a charge. A third option, actually running away out of charge range, is possible but carries large risks as you loseboth your attack ability and your Dex bonuses.
Once you get into the middle levels, this becomes a huge tactical decision and depends strongly on the enemy you are facing. When facing an enemy wielding multiple light weapons, it is often better to move away every turn, even subjecting yourself to both a charge and an attack of opportunity than it would be to face a full attack, especially if you can avoid the AoO by tumbling or some other means. But this can backfire -- a multi-attacker who can use skirmish damage, or who has a feat like Powerful Charge, can be as dangerous or more when charging as when full-attacking.