As a Linux/Ubuntu newbie, what is LVM and what is it used for?
In terms of a web server installation, what benefits does it provide?
Would you recommend using it?
You can think of LVM as "dynamic partitions", meaning that you can create/resize/delete LVM "partitions" (they're called "Logical Volumes" in LVM-speak) from the command line while your Linux system is running: no need to reboot the system to make the kernel aware of the newly-created or resized partitions.
Other nice features that LVM "Logical Volumes" provide are:
While being most useful on server systems, I think that features 1. and 3., combined with LVM's ability to create/resize/delete LVs on the fly, are quite handy on desktop systems as well. (Especially if you experiment a lot with the system.)
Of course, all of this comes at a price: the initial set-up of LVM is more complex than just partitioning a disk, and you will definitely need to understand the LVM terminology and model (Logical Volumes, Physical Volumes, Volume Groups) before you can start using it. (Once it is set up, using it is much easier, though.)
Also if you use LVM across hard drives you may loose all your data when only one drive fails.
Reccommend reading the LVM HOW-TO from The Linux Documentation Project.
The main benefit from using LVM is if you have more than one harddrive. With LVM you can group the hardrives into one huge one. Also you can add more space to this group if you add more harddrives.
With LVM you can simple work like you have only one single huge harddrive. Despite that LVM supports a lot expert features.
Without answering your question directly (which the other posters already did), there is an easy answer about whether or not you need LVM: If you don't know some feature during installation in detail, leave it at its default setting. The default configuration will be fine for most users (including me and probably you). :)
What kind of applications or content are you planning to host ? If it is a personal server or something for a small organization, you probably can get by without using LVM.
LVMs are useful if you need partitions etc across multiple disks. I doubt you would need it, given that you're asking here regarding it :)