Battery gone lose everything?

If the battery dies while you are using the laptop, you may lose whatever you are currently working on, depending on things like Time Machine or periodic saves by an application. Data saved to your hard drive will not be affected, since the system normally goes to sleep when the battery capacity goes below a minimum level.

There is the possibility of data loss if there is a hard crash, for example, while running on battery, you remove it when the system is writing to the disk - but this would be the worst case (you do have backups, don't you?).

Your battery question is answered, but since you say you are new to Mac, you should be aware that there are some very easy ways to back up and stay backed up. One is with Time Machine--you mount an external drive while you are working, and your work is automatically backed up as you work by Time Machine.

Another useful way to back up is to create a bootable clone on an external drive using a cloning utility like Super Duper or Carbon Copy Cloner:

http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

http://www.bombich.com/software/ccc.html

You can download either one and use it as long as you like for free. If you want to unlock some of Super Duper's more useful features like Smart Update, you can register a copy for about $28. CCC is donation ware.

If you have a big enough external drive, you can do both by partitioning it with one partition for Time Machine and a second partition for your bootable clone.

Both Time Machine and a cloning utility will back up everything you have on your hard drive, and not just specific individual things like photos.

Nobody has ever succeeded in repealing Murphy's Law, but a good backup strategy will save you from the worst disasters.

Good luck!