It goes by many names - FID, EID, EIN - whatever the abbreviation it is, this magic 9 digit number is to your company what your social security number is to you. This is how the IRS will track all of your company taxes. It is required for corporations because most states require it on your Articles of Incorporation. Any company, however, must have one to legally employ people. Additionally, you'll need one if you want a corporate bank account. How do you get it? It's actually really simple.
The Employee Identification Number Application can be found on the Internal Revenue's Website here. There's a link about halfway down the page that says "Apply Online Now." Don't blow by the instructions and the FAQ on the above linked page, however. There's a lot of useful information there.
The only point I really hit a snag with is on item 8a - If you're a C-Corporation, you will fill out an 1120 form. If your'e an S-Corporation, you will fill out an 1120-S form.
Immediately after submitting this application, you will receive what's called a provisional EIN. More than likely, this will be your EIN if your application gets successfully processed. You should get an official notice in the mail within a few days. The number that you receive will be utilized when you go to file the next important document - the Articles of Incorporation. I'll discuss that form Part IV.