From BlenderWiki

Distributed Render Farm

There are several levels of CPU allocation that you can use to decrease overall render time by applying more brainpower to the task.

First, if you have multi-core CPU, you can increase the number of threads, and Blender will use that number of CPUs to compute the render.

Second, if you have a local area network with available PC's, you can split the work up by frames. For example, if you want to render a 200 frame animation, and have 5 PC's of roughly equal processing power, you can allocate PC#1 to produce frames 1-40, PC#2 to frames 41-80, and so on. If one PC is slower than the others, simply allocate fewer frames to that PC. To do LAN renders, map the folder containing the .blend file (in which you should have packed your external data, like the textures, …) as a shareable drive. Start Blender on each PC and open the .blend file. Change the Start and End frame counts on that PC, but do not save the .blend file. Start Rendering. If you use relative paths for your output pathspec, the rendered frames will be placed on the host PC.

Third, you can do WAN rendering, which is where you email or fileshare or Verse-share the .blend file (with packed datas!) across the Internet, and use anyone's PC to perform some of the rendering. They would in turn email you the finished frames as they are done. If you have reliable friends, this is a way for you to work together.

Fourth, you can use a render farm service. This service, like BURP, is run by an organization. You email them your file, and then they distribute it out across their PC's for rendering. BURP is mentioned because it is free, and is a service that uses fellow BlenderHead PC's with a BOINC-type of background processing. Other services are paid subscription or pay-as-you-go services.