Frigates

Frigate is first class of 'real' military starships, capable of FTL travel and more 'proper' fighting, without (in most cases) being forced to attack en masse, or being a ship that was somehow depended on being assisied by ships of another class to act. They were, however, much more often fielded at the beginnings of human space travels and exploration.
Over time, with technological improvements that allowed to field bigger and bigger ships, frigates lost most of their military potential. Today they aren't used by regular fleets in other ways than scouting - they are small and fast, and are perfect to jump into hostile systems to scan them before retreating to bring informations to rest of the fleet. They have enough place on board for decent sensors, while still being light and thus - cheap to use.
Every proper military units will rather easily rip them apart, so there is no place for them on a battlefield, and their AV defense usage was taken by destroyers and groups of corvettes devilered to the battlefields via voidcraft carriers. Although it is the truth, that there are still fleets that used them the latter way, but that's definitely rather marginal.
Although, they are extremely widespread throughout Galaxy. But, as a ship of police, pirates, and various individuals, in whose case they are local equivalents of private jets of XXI century, just armed, much faster and capable of bringing a person from one planet to other, even in a different star system. They are also frequently used by various paramilitar forces and revolutionary governments, since they are easy and cheap to buy and do not require too much people to crew them. They are also sometimes used as training ships when some state is barely beginning its fleet creation.
Due to them being regular military units capable of hyperspace travel, they are MUCH bigger than corvettes. Reason is simple - hyperdrive is big, and unlike courier corvettes you also need to put a bigger crew inside, decent armour and weaponry.

Variants

Armored Frigate - A singular type of frigate that does not possess hyperdrive, and with its engine severely reduced... with the space that was thus freed filled with armour and at least a single heavy cannon capable to damage a destroyer. They were created for patrol service in the system. They are quite good as a police ships, that can easily support the instalations on the outer borders of system in case pirates attacked. Also, they can't be refitted into variants.
Microcarrier - A very interesting frigate variant. Microcarrier is a assault frigate devoid of majority of its inner elements, with heavily weakened weaponry, instead capable of fitting inside a squadron of fighters/bombers (in most cases, light ones). Due to lack of place they do not exactly start from them, but rather are ejected, and then they are 'sticked' into outer hull parts, delivered to enemy by their carrier unit. Return is long and ardurous, that's why most of them also possesses some sort of ammo and fuel ejectors. In its entirety this is an interesting construction, although regular fleets prefer using voidcraft carriers, escort carriers and supercarriers. Microcarriers are a domain of pirates, mercenaries and so on

Configurations

Scout Frigate - Frigate configuration practically devoid of weaponry, instead fitted with much better engines and powerful sensors. While typical sensors allow to detect a fighter on an opposite end of the system, an expansive sensors of scout frigate allow to distinguish for example military units pretending to be civilian (and vice versa), at least in some cases. Their mission is to jump in, scan the system, and flee as fast as they can.
Courier Frigate - Frigate configuration with limited weaponry and powerful engines and comm-systems. Not to mention a notable fuel tanks, if they are powered by fussion reactors. They are, in fact, a heavier version of courier corvette, used by variery of military forces... but also civilians (as postal ships, micro-cargo vessel to carry small amount of expensive or perishable, or passenger ships for VIP). They have no other uses.
Assault Frigate - Frigate configuration that can be considered a base one. Decently armed (in light artillery and conventional rockets, but still), quick and FTL capable, but with at best mediocre resistance to hits and pathetically underarmed when is supposed to attack destroyers (or stronger ships) unless with big numerical advantage. It can also be safely named as pirate frigate, as she was extremely often used by pirates and various miscreants, since it is small, fast, capable of catching up to and forcing to surrender any type of cargo vessels - a dream of every pirate.
Missile Frigate - Frigate configuration dedicated to attacking cruisers (or supporting assaults on capital ships) by launching wave after wave of Ship Killer missiles. Practically speaking most of them has at best 20-30 of them aboard, so its at best few salvoes of decent density before running out of amunition. They are oftenly used in typical corvette wolfpack tactics, with whole squadrons targetting a single enemy ship. They are quite decent, but... this is still a highest level of desperation to attack cruisers and capital ships with frigates.
Artillery Frigate - Frigate configuration dedicated to supporting ground forces with fire from low orbits, parmanently changing its position to avoid counterfire. Contrary to the name equally good in terms of artillery and conventional rockets, which they use to chase off fighters off themselves, and their ground forces. Used practically only during planetary assaults, with most countries using corvettes instead.

Power Generation

Fission Reactor

Propulsion

Rocket Engine or Ion Engine.

Beam

50m.

Length

150m.

Height

50m.

Weight

~10 000 tons.

Complement / Crew

30-40.

Cargo & Passenger Capacity

Up to 2000 tonnes.