Vex is robotics design system that helps make it easier to build, use, and program robots and electric vehicles, and can be a very effective method of going about events such as Robot Arm and Robo-Cross. Vex sells there kits with prefabricated plastic and metal parts and motors. The more advanced starter kits also come with sensors and a programming kit. The devices built with the kit can be remote control driven like in Robot events or programmed like in Electric Vehicle.
Here's a basic rundown of the kits Vex sells.
|Radio Controlled or Programmable||Radio controlled + programmable||Radio controlled|
|Number of Parts||500+||300+|
|The Parts include||Programmable micro-controller (Requires Programming Kit), Three variable speed motors, Multiple gears, Various wheel types, and a Configurable chassis||Four variable speed motors, Multiple gears, All-terrain tires, and Preassembled modules make VEXplorer easy to build|
|Other Features||Unlimited design possibilities, Operate multiple robots at the same time with alternate frequencies, and User Inventor's Guide gives step-by-step instructions||Spare/Replacement parts sold separately, Some components compatible with original VEX systems, No Programming Kit Needed, Includes FREE SolidWorks Student Design Kit, XP compatible only.|
Spare Parts and Accesories
If you have either of the above kits but need more metal to make your competition robot there are metal pieces to buy here  There is an advance metal pack also.
Robotics Fasteners and Parts
If you need more fastners then go here to get Vex compatable standard thread size fastners and other accesories. 
Robotics Cables & Accessories
Go here if you need a longer cable, more jumper plugs, spare motor cluthces (good idea because the tend to get striped), a drive chain, or anyting else like that go here 
Starter Kit Replacement Parts
This link will take you to a page with replacement parts. You can use this page to buy replacement parts or order extra parts. 
Programming and Sensors
If you want to get a more advanced programming kit or you need more sensors or different sensors Vex hae a large selection here