Split build is an effort to provide OpenOffice.org buildable by pieces; and additionally the way you can see with all the free software projects that use autotools - just ./configure ; make ; sudo make install.
But what I mean by 'building by pieces'? Imagine you are completely new to OOo development, and notice a tiny misbehavior in Calc. What do you have to do now? Download all the giant monolithic sources, guess the right configure switches, install all the dependencies, learn the OOo build system, and do the build. If you did not get demotivated by now, you might try to fix the original problem.
ooo-build makes it a bit easier - it has default settings for lots of the distros out there, and automates the download of the sources. But the ultimate answer is the split build :-)
With the split build [when finished, of course ;-)], all you'll have to do is install the OpenOffice.org-*-devel packages using the way usual for your distro (in openSUSE, it will mean zypper source-install -d OpenOffice_org-calc, in Debian apt-get build-dep packagename), clone the Calc tree, and do ./configure (or ./autogen.sh to generate the autotools stuff) ; make .
At the moment, it is work in progress, taking place in the ooo-build git repositories. So far, bootstrap (the common build pieces), ure (UNO Runtime Environment), libs-extern (3rd party libraries), and libs-extern-sys (3rd party libraries, usually available in recent Linux system) are in usable shape, more to come soon.
Oh yes, and for those who still want to build everything, there will be a possibility to do make world in bootstrap that will build everything from the beginning to the end :-)