Where can I find out what productions are being filmed in my town?

I couldn’t begin to answer that question because it’s different for every state. I’d suggest that you begin by looking to see if your state has a website for their Film Commission then look for a telephone number which may (or may not) be labeled as a “hotline”. People in the state of Georgia should dial up the Film & Videotape Office hotline.

How much do extras make ?

The last time I checked an extra could earn anything from a hotdog and a slap on the butt (on the low end), up to $100/day and very polite treatment (on the high end). I don’t the going rate, and it varies from town to town, but don’t be surprised if it’s not all that much. Remember: you’re doing it for the experience.

Under no circumstances pay a company to teach you “how to be an extra”. I consider this unneccessary, and many would say unethical. I have firsthand experience with a company called “Reynolds” based in Atlanta who were offering (for a fee) to teach people “how to be an extra”. And if they found you work as an extra they would extract a portion of your daily earnings as an “agent’s fee”. I hope that they have discontinued this practice, but take them to task for having deployed the practice in the first place.

How can I be an extra?

First you’ll need to find out if there are any movies filming in your town, then you’ll need to call them to find out about casting calls for extras. Often there will be a separate telephone number for extras casting, and there will only be a recording on the other end of the phone. DON’T try to call the main number back and pester the person answering the phone because the word will spread that you’re a nuisance and you’ll never work in this town again.

What is a grip (gaffer, best boy, etc.)?

Not that you couldn’t find the answers anywhere else on the internet, but here’s a quick rundown:
DP = Director of Photography
Grip = they build camera rigs, create shadows and drink beer
Gaffer = Key Electric or Chief Lighting Technician
Best Boy = 1st Assistant to department head (grip or electric)
Dolly Grip = the grip who operates the camera dolly (not easy)
Key Grip = the Head of the Grip Department
Focus Puller = the person who focuses the camera during a shot
Loader = the person who loads (and unloads) film magazines
2nd 2nd = the assistant director usually in charge of basecamp

These answers have been simplified, every one of these jobs has many aspects that cannot be encapsulated and the interaction between departments adds yet another layer of depth that is constantly adaptive dependent upon the people filling the jobs.