To answer your question we'd need to know a lot of things.
What kind of video? The scan/encoding system: NTSC, PAL, HDTV (720p50, 720p60, 1080i50, 1080i60, etc.)? The format: composite, S-Video, HD-SDI, HDMI, DVI-D, DVI-A, component, etc.
How many frames do you want to capture?
How long do you want to hold them?
What format are you going to view/print/operate on them?
How and to what are you going to transfer them?
A device/circuit that captures single fields (not the same as frames!) of composite NTSC and displays them as composite NTSC is very simple, while something that needs to take in 1080p50 HDMI, store it in a removable indexed drive, and transfer it to a computer as some proprietary format is going to be a much more complicated project.
