Windows

For windows you should probably use the "dshow" input source. See  here. You can also possibly use the (now out dated) [vfwcap] device.

Linux

On Linux, we can use video4linux2 input device to capture live input (such as web camera), like this:

ffmpeg -f video4linux2 -r 25 -s 640x480 -i /dev/video0 out.avi

If you need to set some specific parameters of your camera, you can do that using  v4l2-ctl tool.

You can find it in ubuntu/debian package named v4l-utils.

Most probably you'll want to know what frame sizes / frame rates your camera supports and you can do that using: v4l2-ctl --list-formats-ext

Also, you might want to correct brightness, zoom, focus, etc. with:

v4l2-ctl -L

and

v4l2-ctl -c <option>=<value>