Ticks are the small yellow and orange enemies that you usually see chasing you around the map and trying to cling to you. The easiest way to go about killing them is to pick them off with explosives before they get close enough to cause some real damage, though they aren't really a threat until inferno.
A couple good methods for farming these kills would involve camping the bomber spiders (The giant versions of the ticks, that spawn them by shooting them out) as well as running the first stage on survival up to wave 5. The reason for this is the fact that this wave is nothing but ticks and with a missile launcher or your weapon of choice this should be made easy! But don't forget about killing ticks yet, there's also an achievement for 5,000 kills. This is just the beginning!