No it's not, because Yellowcard is only slightly good at what they do. Uhn...asking what ska is...it's kind of hard to compare it to other genres.
Basically it tends to be very upbeat with off-the-wall rhythm in it's lyrics and brass instruments always play a part, lots of times with loud fanfare type blasts. The best ska music I've heard is from the band Bandits of the Acoustic Revolution. Reel Big Fish is pretty good, but some of their stuff is really boring.
But like other genres, don't just listen to one ska song and judge it, listen to a variety from different artists (ska doesn't seem to have too many, though).
But uhm, no, it's not too much like punk.