OK, I got it. I just noticed that your profile says you're on Boost so now it makes sense.
Let me see if I can break it down a little for you.
The kernel is the core of the operating system. It is the part of the ROM that controls the CPU. Custom kernels give you more control over how the CPU operates. "OC" means Overclocked, which means that the CPU is running faster than stock. The developers can also undervolt the CPU at the same time they overclock it, meaning it runs faster and uses less power. Less power consumption = longer battery life. So the two main benefits of a custom kernel are your phone is faster and the battery lasts longer.
As far as EXT4 goes, that is a little more complicated. I'll try not to go too far down the rabbit hole with the technical stuff, but basically it refers to the OS file system. If you know anything about Windows computers, you know that the two fie systems are NTFS and FAT32. Android phones are based on the Linux operating system, and they can either be RFS or EXT4. RFS is the stock file system currently used for Eclair and Froyo. Gingerbread is based on EXT4. If you do a little research you can read all about the differences, but in a nutshell EXT4 is faster.
Clockworkmod 2.5.5 is the latest version that supports RFS. CWM3 is the latest version that supports EXT4. Once you decide which direction you want to go, its an all or nothing decision. If you stick with RFS you must make sure to only use ROMs, kernels, themes, etc that are scripted for RFS. If you decide to go with EXT4 then you have to make sure that any ROM, kernel, theme, etc that you want to flash is scripted for EXT4.
Which one you choose is up to you. There are lots of arguments on both sides, so do some research and choose what is best for you. My personal opinion is if you are a tech junkie like me and want to keep updated with the latest and greatest, go with EXT4 because that is where Android is headed in the future. If you just want your phone to make phone calls, check your email and occasionally surf the internet, stick with RFS. There is plenty of support for both right now.
Here are the links to what you need. You are going to have to register with xda to download anything, but you might as well anyway. There are a lot of great resources for the Epic over there.
One Click Root & Recovery for the Epic 4G! v2.5.5 Now has ClockworkMod Redirector! - xda-developers
[MOD][One Click Root] Clockwork Mod version 3.0.0.5/6 - UPDATED FOR EB13! - xda-developers