If you have a cellphone with a monthly limit on how much data you can use, here are some tips on what types of phone use will gobble up your precious megabytes:
Streaming video and videoconferencing
The biggest offender. One minute of YouTube-quality video eats up 2MB. If you're on a plan that gives you 200MB per month, you can't even watch Lady Gaga's "Telephone" video once per day. If you're on a 2GB plan you can, but don't make your iPhone a replacement for a TV. In either case, it's fine to indulge in YouTube and Netflix if you're on Wi-Fi.
The second biggest offender, and potentially more serious. While video is something we need to see to enjoy, Internet radio is more of an accompaniment to other activities, such as jogging or doing dishes. That means some people like to keep it on for hours. Audio consumes about a quarter of the data that video does, but 10 minutes a day will break the bank if you're on a 200MB plan. One hour a day of Pandora consumes nearly 1GB, which you can afford if you're on a 2GB plan and don't use other data-hogging apps.
If you're a real shutterbug, photos can consume significant amounts of data. Sending and viewing photos both count toward your monthly limit. Posting 10 photos per day eats up most of a 200MB plan. If you're on a 2GB plan, you probably don't have to worry about photos.
Navigation apps consume lots of data when they retrieve map images, up to a megabyte a minute. You're also likely to use them for long periods of time when you're away from Wi-Fi, such as when you're driving. Watch out for these.
Web pages vary widely in size, so this will depend quite a bit on whether you like to visit graphically rich sites (lots of data) or spare, text-oriented ones (less data). But roughly speaking, ten pages a day will eat up about half of a 200MB plan. Again, those on 2GB plans don't need to worry much about surfing.
Roughly equivalent to Web surfing. Status updates won't take much data, but sending photos and viewing friends' pictures will.
Most emails are tiny, in terms of data. Basically, you can send and receive email all you want, as long as they don't have attachments such as photos.
Like email, these short messages don't use much data, but if you follow a lot of people and click on links, usage adds up.
Small, focused apps that report simple but useful things, such as the weather forecast, save data (and time) compared with looking up the same information on a Web page.