Right, so a flashing a different ROM won't necessarily improve battery life. A "ROM" is basically the Android OS, and everything that it needs to boot and run. You're running a ROM right now, in fact. It just happens to be the stock one. Some developers out there, however, will take either the stock ROM, or even source directly from Google (AOSP), and modify it to their liking. These tweaks can involve anything from performance to theming. A ROM is basically the entire system, and a custom ROM can have anything from minor to major changes.
A kernel is part of a ROM. It is basically the behind the scenes of any operating system. This is where you will see most battery savings. You can flash a custom kernel without flashing a custom ROM.
Here is a good kernel guide for you to read. You can read about the different terminology and how different kernels can save battery life. I'm sure if you ask or read around here, you will see a lot of recommendations for netarchy. Read that guide, and we can guide you from there.