There are many file formats out there—PDF, JPG, GIF, PNG, AI, TIF, and on and on. It's enough to make your head spin, and they each have their own purpose. The good news is, you don't have to worry about most of them. Here's a quick primer on file formats:
AI (.ai)—This is an Adobe Illustrator working file, so it's a vector file (we talked about those last time). If you send this to your printer, he will be happy to get it. Keep in mind, unless you've outlined your fonts (made them into shapes instead of editable text), you'll need to supply those to the printer, too.
PDF (.pdf)—This is Adobe's proprietary Portable Document File, developed in the '90s as a way to make sure the document you create and email to your Microsoft Windows-using friend looks the same on his computer as it does on yours. PDFs can be made from lots of different applications—Illustrator, Photoshop, InDesign, etc. They're nice because a. they can be marked up and edited for revision, and b. if it's made from a vector application like Illustrator, a printer can open it up in Illustrator without much trouble. Pretty handy things to have around.
JPG (.jpg)—A jpg is a flattened, raster image (that means it is made of pixels; read the previous blog post for more information). Many of the images you see online are jpg (pronounced jay-peg). You can export a jpg image from an image editing software app like Photoshop, and you can adjust the image quality to get the size you need. So, if you have a particularly large image, you may set the quality to 20%—it's going to be less distinct, less crisp than at, say, 80%, but the file will be much smaller. If you save a jpg as a jpg, it will be even less distinct, until, like making a copy of a copy of a copy, it's just a blurry mess. Anyway, the jpg format is generally best for photos to be viewed online.
GIF (.gif)—This is also a flat raster image, but it is made by selecting a set number of colors out of a possible 256—the fewer colors you use, the smaller the file size. So an image using 256 colors will be larger and more accurate; one using 16 colors will be much smaller but will look less like the original. Gifs (pronounced "giff" or "jiff," depending on whom you ask) are best for simple images with a smaller range of colors or flat areas of colors; they can also be animated, which is neat. Sites like giphy have zillions of animated gifs for any occasion.
This is a pretty good start to file formats you'll encounter out in the world, but let us know if there's anything we missed or you'd like more information about, I can talk about this all day.