The game consists of two sides, the aliens (also called Kharaa) and the marines and they are quite different to each other. The marine players have to build the buildings that the commander creates, whilst the aliens can help build a building faster it is unnecessary. Extra abilities, weapons, classes etc. will need to be researched by the commander before you can use them. Aliens evolve into completely different lifeforms and can have a choice of what upgrades they take as these lifeforms. The basic alien unit is a dog like creature that can climb up walls and is primarily a melee unit. In addition to this you have a support class, a flying alien, a stealthy (and very deadly) one and the massive Onos which is a powerful meat shield.
The marines handle upgrading differently. Firstly there are universal upgrades that apply to you no matter what, which go straight to your weapons and armour. Then you get to choose what weapons and gear you take from an armoury or prototype lab. The armoury gives you access to the shotgun, grenade launcher and flamethrower as well as the welder (which lets you repair structures and your teammates armour) and mines (which blow up stuff). The prototype lab gives you access to jetpacks and the powerful Exosuit. The Exosuit is a robotic suit of armour that comes equipped with a minigun and can be upgraded to wield two miniguns making it hugely powerful.
Whilst balance is a contentious issue and the developers are constantly tweaking it in general I felt the game was quite balanced and either side could win it despite the large differences between them. Team work is something that is very much required and a good commander makes a huge difference too. Not only does a good commander build the structures you need in the correct order but they'll also tell you where to go and who needs help. Another issue the developers have just addressed is that of getting new players up to speed. They have included tutorial videos that play whilst you are waiting to respawn that tell you tips and tricks. Accessibility was something the game was criticised about by a lot of reviewers but it has become much less of an issue now.