Fight - This is the default action in battle. The character will use whatever weapon they have equipped and attack any enemy you select. If they're not holding any weapons, they might try punching, kicking, or biting depending on the type of character.
Right before pressing Enter to select your target, you can refine your aim by pressing and holding 8, 4, 5, 6, 1, or 3 keys. Consider how the keys are arranged on the numpad and you'll be able to work out roughly where you're aiming. 8 aims at the head of the target, 4 the left side, 6 the right side, 5 is the center, 1 is the bottom left side and 3 is the bottom right side. Try disarming your enemy by striking their hands, or knocking them over by striking their legs. Different targets apply for different locations depending on the creature. The left side on a humanoid might strike the left arm or hand. The same location on a wolf would be the left legs or left paws. Use this information to your advantage.
Magic - (Only characters that have magic.) Cast a magic spell. Only spells that can be used in combat will be selectable. Casting magic uses magic points.
Techniques - (Only characters that have techniques.) Uses a special ability. Abilities use ability points and can also add extra delay before your next attack.
Defend - Brace for incoming attacks. Defending grants a small bonus to deflecting enemy attacks. It's faster than a normal action.
Throw - Throw a weapon or item at an enemy. Throw allows you to use anything in your inventory as a weapon. Most items make poor weapons, however.
Get - Pick an item up off the ground.
Equip - Put on armor or wield weapons. This doesn't use an action.
Inventory - Use an item from your inventory. This allows you to use potions, scrolls, and other activate-able items in combat.
Flee - Attempt to escape from battle. Success depends on your fastest character versus the fastest enemy character.