Turrets, Single Cannons, Dual Cannons and Dual Heavy Cannons all fall under the category of cannons.
Beam Arrays, Dual Beam Banks, and 360 degree Shuttle Beams are all in the category of beam weapons.
Interestingly, I have seen people assume that Cannon: Scatter Volley just fires in an arc around your current target, but on a Turret, it has a similar effect to Beam: Fire at Will. It's not exactly the same, but close enough in function. I know I use it.
As a slight aside, what is the reason for cannons and turrets being unable to target subsytems?
Not speaking for the devs but my guess is:
1) Encourages Science Vessels to stick to beams. (Cannons are more for Escorts, obviously, and to an extent Cruisers)
2) Makes up for beams' lesser damage, due to arc of fire. (Gets back to 1, although Science Vessels ended up getting Sensor Analysis as well, so apparently it doesn't help compensate for low damage THAT much)
3) Prevents Escorts from making full use of it even if they gain the skill with a BO.
4) Targetting a specific subsystem requires precise aiming and a fast travel time weapon. Beams strike the target sooner than energy bolts, and the point of impact can be used to adjust the aim continuously. The energy may also be built up on the target more gradually.
5) In the canon, Federation ships loved to target subsystems as a non-lethal method of defeating a foe, and they all tended to use phasers. So the connection was made between non-lethal tactics and the beam weaponry of the Federation. (Note also phasers seem to have a "stun" setting but blasters typically don't)