A boot-loop can have different reasons. Though in most cases this happens after flashing a new ROM, other things might affect it as well: e.g. some system related apps which have services or the like started quite early. But those special cases usually rather result in a force-close-loop than in a boot-loop.
There are some things you can try before asking a question, so we don't need to explain the same things over and over. Please make sure to include results with your question:
See whether you can boot your device into Safe Mode. How this can be done depends on your device, as different devices use different key-combinations to achieve this. You might want to consult the article How To Use Android's Safe Mode to Save a Phone from Boot Loops for details. For a quick go, the main paragraphs are:
On Samsung’s devices, almost all users will be able to access Safe Mode with by pressing and holding the menu button during start up. If it’s a capacitive menu key, make sure to release and press again each time the LED under the key comes back on. You don’t want to miss the catch for Safe Mode because the button released itself.
The Nexus One is easy, just press and hold the trackball while booting. HTC’s other phones are a little harder. On HTC phones without physical buttons, turn the device on, and wait for the HTC logo to come up. At that time, press and hold the volume down toggle. For other devices with physical buttons, HTC or other, try pressing and holding the menu button during boot.
If you successfully reached Safe Mode, you will see the string "Safe Mode" displayed on your screen, usually in the lower left corner.
If that works, it is one of the apps you lately installed which causes the boot-loop (otherwise you would not have completed the boot this way). So go and remove the app (via Settings→Apps→Manage Apps), reboot, and you should be fine. In case you get problems leaving Safe Mode: Power down, take out the battery for 30s, insert it again, power up.
If Safe Mode is not reachable either, you can try booting to recovery mode. Again, how to do this depends on your device: different key combinations are used. Most common combinations are:
- Vol++Power+Home (Samsung)
More hints can be found e.g. at How To Boot the Android mobile device into Recovery Mode or How to Enter Recovery Mode on Any Android Phone or Tablet (using special apps or ADB). In the menu appearing there, select to "Wipe Cache". Reboot.
If not, or if the device has no custom recovery installed, a factory-reset is all that's left to you. This will wipe all your data and the apps you've installed. You can do so from the very same Recovery Menu by chosing "Wipe data" or "Factory reset" (it is labeled either the one or the other).
Problem still unsolved?
If none of the above solved your problem, please make sure to include this fact with your question. State what you've tried, and what the results have been. This not only saves us unnecessary questions -- but also saves you precious time, as we can straightly can forward to the "real problem" with less "guess-work".