TFS uses MS-Build as the build engine. Ms-Build gets the code from TFS code repositiry and builds it in a temporary build location and copies the build result (binaries, web sites, etc...) to the permanent build output location refered as the DropLocation.
To customize my build project, I needed to have access to the temporary build location within in build definition and I finally found the $(BuildDirectory) property which does exactly I want.
During my search, I found Sayed Ibrahim Hashimi's blog with a list of useful "MSBuild reserved properties".
You can also take a look at MSBuild Reserved Properties at Microsoft website.