A word meaning "possessing great physical beauty", naturally being an expansion of pulchritude which itself means beauty, especially of a woman. From the latin pulchr- meaning beautiful. Both words are decidedly out of favor.
In fact, if you search around for uses of this word, you'll primarily find it in on-line dictionaries, and in pornographic spam email. In this latter purpose, it is used because spam filters look for frequently-used combinations of words which indicate that a message is likely unsolicited, and the word does mean beauty however obscure it might be. This leads to subject lines like "Grid-manager mailing list archive: Son fucking pulchritudinous" and "[kde-freebsd] Have you ever seen pulchritudinous Teenie".
Regardless, the only real difference between "pulchritudinous" and "beautiful", besides the difficulty of spelling one of them, is that one of them is longer and more obscure.