Since the first permanent extra-planetary human settlement was established on Mars, the Carrier has remained the best and only tool for human expansion across the stars.
While the original carrier was built without the intention of carrying large amounts of colonist hardware to mars in an economical fashion, it has evolved beyond that. The Enterprise was a scant 280m in length, the Mayflower 512m, and in a few generations, carrier sizes may well exceed 1km. The Olympus, the sole military carrier of the Federation fleet, is over 800m long.
Because the Carrier is the only platform capable of carrying the warp drive, and because the utilization of the warp drive is very expensive, the rest of the carrier has to be built to carry as much as possible with each jump. Thus, modern carriers are slow, heavy, and large constructions.
A modern carrier is, first and foremost, built around the warp drive. The presence of the warp drive instantly requires power generation: Every carrier contains multiple Atomic Blenders, some directly slaved to the warp drive, some powering the rest of the vessel.
To maintain the hardware, a large crew is needed to keep the Carrier functional. The size of the Carrier and the payload determines the size and training of the crew, but tens of thousands of astronauts per vessel is not unheard of. This crew requires life support, power, accommodation, and entertainment. Most carriers will contain a forest of genetically enhances plants that aid in the recycling of air, not to mention dedicated water and nutrient recycling systems.
All this already represents a highly expensive investment. To protect herself, almost all carriers equip a version of the Bubble Shield – a network of ports lining the outermost hull of the Carrier. The foam dispersal system ejects a quickly hardening foam to outer space when an inbound object is detected, thus absorbing the impact. Carriers traveling through micro-asteroid fields will trail a monochrome stream behind them, the foam solidifying in the cold of space. Other protective measures include a thin Plasticrete exterior hull that can be replaced if some debris makes it's way through the bubble shield.
For bigger game, comes the particle accelerator cannons. Drawing from a reservoir of inert xenon gas, these large weapons can be used both defensively as point defense turrets, or pointed at other hostile objects in space. Because the PAC fires highly ionized Xe nuclei, these weapons are drastically reduced in effectiveness in atmospheres.
Finally, the payload. The raison d'etre of the Carrier, the payload determines the purpose of the Carrier. The first Carrier, Enterprise, is a transport, or mercantile vessel: Designed to haul goods from Earth to Mars. The Mayflower was a colony vessel – loaded with early iterations of the colony vessels that now form the backbone of Federation colonies, it was built with the intent of carrying a large amount of humans into the far reaches of space.
However, Carriers tend to be highly unique – Discovery, while the same generation as Enterprise, was a builder carrier – her internals were filled with facilities that could build installations in deep space. It is for this reason Discovery was used to build the hydrogen harvesting facilities around the gas giants Saturn and Jupiter in the solar system.