The LTS releases are expected to be very stable, suitable for offices, and "mission critical systems" like power plants. LTS gets all the "Important Security Updates" and any updates that fixes security issues.
LTS can be also updated with newer kernels, but it won't happen on it's own.
Typically, 12.04 LTS won't get the newest Unity features from 12.10, 13.04 etc. nor will it get newer versions of applications, like OpenOffice, unless such a feature or a newer application fixes a security hole.
Having said that, there are ways to get some of these into the LTS using the backport repositories and PPAs. In my opinion it is not worth the trouble unless you have some special circumstances preventing you to use the latest.
Normal releases on the other hand get the newest features and more recent applications like OpenOffice.
See What's the difference between a Long Term Support Release and a Normal Release? for more details.
To help answer your question about which version you should choose, my advice is the following:
- If you are going to use Ubuntu as a home desktop and either
dual-boot Windows or have another computer in case of emergencies,
choose the latest, say 13.04 in a few days when it is released.
I usually wait 3-4 weeks to avoid the mad rush and the initial kinks
(if any) to get sorted out with the first round of updates.
- If this Ubuntu installation is going to be mission critical, such as
you will lose your livelihood (or lots of money) if this computer
does not work well for a few hours/days, then choose the LTS.
Hope this helps