My battery is a year old, but it has 189 cycles. Why is this? I always use my MacBook with the charger plugged in and very rarely without the charger.
I disagree with the previous answer. The Apple battery is designed to be discharged infrequently, for instance around once a week - this is how to get optimal life out of it (see here) - you can safely operate it on charge for about a week before needing a discharge. Provided you exercise the battery occasionally, it'll be fine.
Also, the battery does not cycle when it is on charge - Matt Love is incorrect. A cycle is defined as the battery dropping below 50% and returning above it - which doesn't happen when running from a charger. Approximately one cycle a week will give you optimal longevity before replacement. My battery is a little over two years old, and on 1-3 cycles a week I still get 80-85% of the original battery life out of it.
Letting it die down as often as possible will not improve longevity - this is a common misconception but in the case of Lithium Ion batteries, more cycles = less life.
First of all, it is very unhealthy for your to keep your MacBook on the charger all the time. It is best to let it charge up and die down as often as possible. Secondly, 189 cycles is normal for one year of usage. The battery is still cycling when it's on the charger, it's just not discharging much of the power it's accumulating which is the unhealthy part.
I'd recommend letting it die down as often as possible and you should be in good shape.