The thing about iMovie is this: you can do a lot with it, but chances are you'll start editing, get excited and, metaphorically speaking, try to stand up and bump your head on the ceiling of the program's limitations. You can either sit back down and try to figure out how to work within the confines of the program you have on your laptop, or you can move on to an editing program like Final Cut Express, Final Cut Pro, or another program. Rumor has it the new, less expensive, rejiggered Final Cut is coming out in a couple of days . . . .
What do you do with digital video? Video can be of anything: screen captures of software demos made into tutorials, a musician playing, a talking head, a video tour-- and "filmed" with a growing range of camera devices including your laptop.
Like any digital media, you can store it locally, on a server, or both (using different formats, codecs, sizes for different purposes). You can shoot a piece of video on many smartphones, trim it, and upload it directly to youtube. You can make a DVD. If video is on youtube or vimeo, then it can embedded easily on a website or blog, by you or anyone else. A DVD or video on a flash drive means no internet connection is necessary; a DVD can play on a television with DVD player, without any computer necessary. As always, the key is to think through how and why you will use the piece of digital media for teaching, and then decide what forms you need. It's better to create multiple versions for multiple purposes as you make learning objects, so you have your digital media accessible in the form you need for whatever purpose or container you want to use at any given time. Then you can improvise around the learning objects you've created and collected.