[T-Mobile Samsung Galaxy S II SGH-T989] kangelov:
Odin is a tool used internally by Samsung. As such, you can't expect Samsung to write you a manual. Be very careful, you can cause a lot of damage with it. It is powerful, though: it can bring your phone back from the brink of death. Or it can take a healthy phone and send it over the edge. What you are doing now is something every rooter does on day 1, and only after days and days of research. I am uncomfortable with what you are going to do now given that you have never done this before, but you have no choice: it's this or a useless phone.

Here's the rundown:

A PIT file is something like the Master Boot Record of your harddisk. Problem is, the first partition is something like your computer's BIOS: it tells your phone how to bootstrap. It can bootstrap into the normal mode, which runs Android, or it can initiate Download mode. Download mode is what ODIN needs to replace parts or all of your phone's software. If something goes wrong with that bootloader, your phone is something called a hard brick: it won't power on, it won't even run Download mode so that you can recover it back to stock. It is dead. If that ever happens, you need something called a JTAG flash: you send your phone out somewhere, and someone takes it apart, hopefully carefully, plugs a programmer into an engineering port on your phone's logic board, and reprograms the bootloader back again). Now with PIT file having the major potential to trash things completely, you really have to pay attention you don't load the wrong one. Problem is, the ODIN you just downloaded DOES come with a PIT file, and that PIT file is WRONG for your device. Follow the instructions! Follow the instructions really really carefully. Don't check off extra things! Don't uncheck things that should be checked! Don't add PIT files! Make sure you load the right file to the right place in ODIN. Your phone depends on it.

ODIN will program your phone from scratch, but you wouldn't want to use it too often. It's a dangerous tool, as you can imagine, and it is meant to replace your phone's software, not maintain it! That's why we have Recovery. Recovery is what you use to do basic maintenance to Android, such as factory reset and clean cache and stuff. With those custom recoveries out there, you gain the added ability to flash custom ROMs and a few other things as well, which is what this forum is all about. So, step one in anyone seeking to root their phone is, flash a custom recovery using ODIN, since you need Download mode for that. In an ideal world, you will never need to use ODIN again after you flash your custom recovery. Well, in the real world, you should needed it only very infrequently.

Your phone first got its custom recovery from a tool designed to automate things for you, and sent you flashing without ever giving you an idea how your phone works. Now you don't know how to recover your phone from a flash gone wrong: in the long run that tool did not do any favours to you. The steps I gave you will bring your phone back to life, it brought mine several times already. Something about those new JB ROMs corrupts every recovery I have tried. I don't know why, I don't know how, but it does it. To make matters worse, ODIN without a proper PIT file cannot repartition your phone and fix whatever mess caused the problem to begin with, so restoring stock directly won't do the trick, you need to flash custom recovery, to fix recovery, and then Superwipe to recreate and realign your system, data and cache partitions: that's a good chunk of your phone. So, after you do this, you should be back in business. From beginning to end, if you know what you are doing, the entire process should not take more than 30 minutes, but don't rush. Take your time: take hours, take days. Take all the time you need.

Read before you do anything, and ask if you don't understand.