Mainly a PvE server with a splash of pvp in certain areas and events. (main area is the CQB room where you can challenge other players to contests)
SoS or Swords of Sanghelios are made up of the traditional covenant species who have split from the covenant and work with humans. there you will have elites, jakals, and grunts.
HoJ or Hearts of Jiralahele is made up of purely brutes, and have much the same thought process behind them. they were hired by the unsc much like a PMC, but mostly remain as an independent organization.
as for rp, they generally do their own thing.
as for UNSC, they have four-5 mini factions inside of it consisting of ONI, ODST, Marines, Naval, and airforce. Naval works with ships, Airforce flies vehicles, ONI does all sort of spook stuff, marines work as cannon fodder for events, ODST generally stick to themselves and do the most serious of the RP on the server
Marines can be a lot of fun if you have a solid leader, but honestly there is a large selection.