A. Windows 7 and Windows Server 2008 R2 introduced a new feature that allowed data downloaded by one person to be shared with peers on the same local subnet, a feature known as distributed mode BranchCache. (An alternative is dedicated mode, which is where a Server 2008 R2 server is specified to cache content for an entire group of computers.) As the name suggests, this is primarily aimed at distributed environments that may have a slow link to the main datacenter, where having 50 users download the same 10MB file is a waste of bandwidth that will mean a poor end-user experience. With BranchCache enabled, the file would be downloaded by the first person to access the file, and the other 49 people will pull it from the machine that already downloaded it.
To use BranchCache, you need Windows Server 2008 R2 to host your content. Your clients must be running Windows 7 or Server 2008 R2, and BranchCache must be enable on both the server and clients.
The good news is that SCCM can take advantage of this functionality, providing you're running SCCM 2007 SP2 or above. You must check the option Allow clients to transfer content from this distribution point using BITS, HTTP and HTTPS on the distribution point properties in the General tab of SCCM. You also need to configure the advertisements to download and execute, instead of running directly from the distribution point.
There's a great MSDN blog that goes into more detail on this topic.