Generals were extremely high ranking military officers in the army, able to command platoons, legions, and hundreds of thousands of troopers.
Usually, in every military force, there are four main categories of the rank: Brigadier General, Major General, Lieutenant General, and General, in lowest to highest ranking respectively. However, since all of them can be referred to simply as "General", the highest form of General is sometimes called a "Four Star General" or a "Full General".
In war times, a Full General could be given the rank of "General of the Army", which would then be considered the highest rank of the entire army.
Every group has their own versions of the rank, such as the Jedi Order, which utilize the rank "High Jedi General" only in times of war.
The navy equivalent of Generals were known as Admirals.