Energy Weapons in Star Trek Online fall into several different categories (disruptors, plasma, phasers, etc), but there are five basic types that are the same, regardless of the type of energy being fired from them. The basic trade off with energy weapons is firing arc versus damage. Weapons that do the most damage tend to do so only when firing straight ahead, while those that have wide firing arcs allowing for broadside (turning sideways to fire front and rear energy weapons simultaneously) tactics tend to have limited individual DPS.
Energy weapons tend to be best against shields, though the more powerful options can tear through a ships's hull quite quickly.
The most powerful energy weapons are the Dual (Phaser, Disruptor, etc) Cannons. These weapons are limited to non-science ships only, and have a tiny, 45 degree firing arc, pretty much limiting you to firing straight ahead. Dual Cannons do the most overall DPS, but you tend to take a beating on the forward shields while using them, as you can't turn the ship and keep firing on the enemy.
Second in overall DPS are the Dual (Phaser, Disruptor, etc) Beam Banks. These beam weapons have a slightly wider firing arc, 90 degrees, and are accordingly a bit easier to keep trained on an enemy while you maneuver. You're still limited to mostly frontal assaults with these weapons, however.
Third in overall DPS are the Cannons. These projectile-firing weapons have a much wider 180 degree firing arc, allowing you to fire straight ahead and to both sides with equal effectiveness. Cannons have limited broadside potential when combined with Arrays that overlap their firing arc, but this can be a bit tricky, as the angle has to be almost perfect.
Arrays have the second-lowest DPS, but make up for it with their wide 250 degree firing arcs. With Arrays equipped both fore and aft, it's very possible to keep your ship sideways and be constantly firing all of your Phasers simultaneously. So, while their individual DPS may be low, the fact that you can fire twice as many at once makes them an invaluable addition to many ships.
Lowest in terms of overall DPS are the Turrets. Correspondingly, they have a full 360 degree firing arc. Many players who equip Cannons or Dual Beam Banks with forward-fire only equip Turrets in their rear slots to augment their overall forward-firing potential. You can slot Turrets anywhere you want, front or back, it doesn't matter because they can strike at any angle.
Equipping too many energy weapons can drain your weapon power very quickly. Keep this in mind when setting up your ship.