To continue on Joerg's track, maemo is based on Debian linux, with it's own custom desktop environment: it's just linux that runs on a device with phone capabilities. Have a read of the wiki to try to get a better understanding of the n900 and maemo.

Even if you install something that prevents it from booting you can use the flasher to boot up with a rescue kernel and you can fix the issues, just as you might with a PC/laptop using a bootable USB disk.

Alternatively you can flash a firmware (or what you consider a ROM) to revert back to factory conditions.

BackupMenu gives you the option at boot up to boot something similar to a Rescue kernel, without mounting the root partition, and allows you to back it up. You can then boot normally and copy that backup on to your PC or if you used an SD Card, you could just pull that out and keep that safe.
If you then need to go back to that backup you just reboot the device with the keyboard open, and when BackupMenu starts up, you just select the option to restore a backup.


This can be a bit overwhelming, but there's so much documentation about this device it will do you the world of good just to spend a few hours reading around the wiki.

And it's well worth doing too, I bought mine in 2010, and before that I'd dabbled with linux, but failed to stick with it (not having a compatible modem was just one issue), but once I started using maemo, I quite quickly crossed over and haven't looked back