Classic Game Mode (classic) is the 'proper' mode of Natural Selection. The easiest way to disguniush a classic map is to look at the prefix of the map, if it is classic it will be 'ns_'. eg 'ns_veil'.
Classic is considered harder to play, especially due to all the added features and the fact that it involves a lot more teamwork.
Resources, commonly called 'Res', is the currency in this mode. To collect Res, the teams must either kill each other, or build Resource Towers on a Recource Nozzle, commonly called a 'Node'.
Resources is the most important thing in Classic, apart from the Command Chair or Hive.
Controlling Nodes are what the game revolves mainly around, because this gives you access to more Res, and therefore better equipment or lifeforms.
When two Nodes are next to each other, or in the same room, it is called a 'Double'. Doubles are very important in the maps they are included in. Because controlling this one room (normally in the centre of the map) will give you a hold on the map.
Commander Mode Edit
For the Marine team, this mode completely changes their gameplay. The biggest change is the Commander or Comm. The Commander does exactly as his name would suggest, he commands the Marines. He also looks after Res and is the only one capable of using it on the Marine team. He is responsible for placing Structures, while the Marines have to build them. He also is responsible for distributing weapons, health packs and ammo packs. He also has to upgrade the technology for the Marines, like armour, weapon damage and Motion Tracking.
There can only ever be one Commander at a time, although Marines can switch being Commanders during the game. Commanders can also be kicked out of the Command Chair, if the majorty of the Marines vote for this.
To enter as a Commander, simply 'use' the Command Chair, although it has to be vacant.
Commanders see from a top down view, and can see all of the map, although they cant see Aliens or Alien structures if a Marine can't, this acting effectively as a 'Fog of War'.