Hi Soozka, I use OS X as well and there are some v handy features and backdoors that I discovered via internet search.
For screencaps, hold down Command (or Apple sign) + Shift, then press 4 and a symbol resembling a crosshair rifle sight should appear. Let go of Command/Shift/4. Using the mouse, lasso the image you want to capture, then let go. A .png should automatically appear on your desktop.
For grabbing webpage content in Safari (.flv, .mp3, .mp4, .jpg, you name it) go to Window / Activity. This launches a list of the files on the webpage and all you have to do is double-click on the desired file (in the case of video, it will automatically start downloading). It doesn't work in all cases, esp. with very strong encryption, but overall is amazingly convenient.
Thanks Heidi - I was going to answer via PM, but, couldn't have said it better myself - plus I only have a mac... so wasn't sure if my answer would have been correct anyway.
Also, thx for the video info - I had no idea!!! Quite frankly, the fact that I can capture an image is a small miracle, as I'm pretty computer illiterate.. lol