JuiceDefender saves battery power (lots of it!) by controlling the device data connection and/or WiFi.
You can schedule regular APN/WiFi activation to let background data sync occur and have APN/WiFi enabled while the screen is on. It also helps in minimizing distractions.
The Easy Mode is a no-fuss one-click way to let your battery last longer - much longer. Just enable JuiceDefender by clicking on the big button and you are ready to go!
If you want more fine-grained control, try Advanced Mode, where you can configure all JuiceDefender features to your liking.
There are 5 triggers for the enable/disable behaviour:

Battery - when battery level gets low (less than 15%), disable APN/WiFi, and re-enable them when battery level is restored. APN/WiFi will also be enabled while the device is being recharged.
Schedule - regularly enable APN/WiFi for a short period of time, to let background data sync occur (email, Twitter, Facebook, stock quotes...). If Quick is disabled APN/WiFi stays enabled for a longer period, useful if your data connection is very slow or you need to sync lots of data.
Night schedule (requires UltimateJuice) - disable APN/WiFi during night time; you can also optionally put the phone in Silent Mode.
Screen - enable APN/WiFi while the screen is on to allow browsing, tweeting, procrastination and general internet-powered enjoyment, regardless of scheduled events and battery level.
Location (requires UltimateJuice) - this trigger controlled by the 'AutoWiFi' button. It disables WiFi when the device is not in range of any known WiFi network. The location is determined via the cellular network, so it's usually quite coarse. It's a fully automatic set-it-and-forget-it WiFi manager!

The priority order of the triggers is 1) location (WiFi only), 2) screen, 3) battery, 4) night schedule, 5) schedule - this means, for example, that when the screen is on APN/WiFi will be enabled even when the battery is low, or that the regular schedule won't occur during the night period.

Note that triggers will only enable WiFi if you have explicitly turned it on - if you turn it off manually, your decision will be remembered and no trigger will turn it on again (until you re-enable it).

Having data enabled 2 minutes every 15 minutes is sufficient in most situations. A longer schedule interval is useful to minimize distractions when you need to get something done.

There's a Home screen widget that shows how much battery life was improved by JuiceDefender over the last 48 hours - what's the best multiplier you can get?
It also lets you enable/disable JuiceDefender super-quickly (just tap on the glass of juice in the bottom left).

Please note that not all features are enabled in the free JuiceDefender. The free version can control only the data APN, and the available triggers are screen, battery and schedule with frequencies of 15 and 30 minutes.

To get complete control over your device juice consumption please consider buying UltimateJuice - it will enable WiFi and MMS control, the location and night schedule triggers and schedule intervals of 5 minutes, 1 hour and 2 hours. You can make your battery last forever! (ok, that's an hyperbole.)

If you want to check how much your battery life is improved by JuiceDefender, get JuicePlotter now - the best looking and most informative battery graph out there! 

What's New in This Release:

· fixed minor problem in settings backup file
· fixed potential issue with Auto Disable

What's New in 1.8.2 beta:
· due to popular demand, JuiceDefender won't proactively (and honorably) shut itself down when memory is low; it'll just cowardly hope that the murderous rage of Android won't kill it