Charting is taking your basal body temperature every morning, and monitoring other fertility signs, then plotting it on a chart. You can find the charts online.
Your temperature goes up after you ovulate, so you can tell exactly when it happened and when you're fertile. I've found them really useful for knowing what's going on since coming off the pill.
Sounds like your ipod app is the same sort of thing- I just like charting because it gives me proof that I have ovulated and tells me exactly when.
I know what you mean about making plans, I'm trying not to look forward to all the Christmas nights out in case I'm pregnant by then.