What kind of stuff do they play?
Just one type of music: awesome.
To answer your question, their music is really heavy, most sources say they´re a band rooted in death metal with folk/progressive influences. They use growls around 50% of the time these days. But they combine that heaviness with incredible melodies that you cannot get out of your head, and their clean vocals are excellent (and the growling parts too). If you want to give them a chance, here´s a short list of my favorite songs:
Born from Fire
The Smoke
Brother Moon (my favorites from my favorite album Eclipse)
On Rich and Poor (from Elegy)
Silent Waters
Her Alone (absolutely beautiful ballad)
The Castaway (the melody-heaviness combo is very clear here. They have created lots of those kind of unusual riffs that I love. Check out Against Windows for another example)
In the Beginning (the groovy part at the middle and the organ-driven part at the end fit very well)