A baritone uke is a little larger than a tenor. It is usually tuned DGBE like the four highest pitched strings of a guitar instead of GCEA like most sopranos, concerts or tenors.
Whether it is worth buying depends upon your personal requirements and tastes. Advantages are a slightly deeper range which can give a fuller sounding accompaniment for vocals and more fretboard room for big hands. Disadvantages are that standard uke chord shapes are transposed on a bari, which can confuse newbies, and a bigger stretch to fret some chords etc if you have small hands. Bari strings may be harder to find in your local store, but you can buy online.
I've tried a baritone 'ukulele and they're not for me. Some people love 'em.