Ticks are routes you've climbed. Go to any route in the database and you'll see there is a link to add the route as a tick. I personally don't use them because it would immediately show everyone how weak I really am.
luckily in my case the pun wasn't realized, literall or figuratively. kevin, if you want to see a tick, SSV is the place to go. Every single time we have climbed there between mid february and early june, we have had at least 1 of the little bugger on us. it's like the tick capital of colorado.