This term refers to the 'pulse' of the game which occurs every 6 seconds. So if something lasts for 6 ticks, its duration would be 36 seconds.
For the most part, the only things that are measured in ticks are spells with durations. All buffs, DoT's, and bard songs have a predefined number of ticks that they last. Item Focus Effects and AA's can affect the number of ticks.