Windows

dshow

For windows you should probably use the "dshow" (DirectShow) FFmpeg input source. See DirectShow.

vfwcap

Windows users that can't use DirectShow can possibly use the (now out dated) ​vfwcap input device, like this:

To list the supported capture devices, connected to the machine:

ffmpeg -y -f vfwcap -i list

That will give us the list like this:

...
libavutil     50.36. 0 / 50.36. 0
libavcore      0.16. 1 /  0.16. 1
libavcodec    52.108. 0 / 52.108. 0
libavformat   52.93. 0 / 52.93. 0
libavdevice   52. 2. 3 / 52. 2. 3
libavfilter    1.74. 0 /  1.74. 0
libswscale     0.12. 0 /  0.12. 0
[vfwcap @ 01c6d150] Driver 0
[vfwcap @ 01c6d150]  Microsoft WDM Image Capture (Win32)
[vfwcap @ 01c6d150]  Version:  5.1.2600.5512
list: Input/output error

So, we can try to grab something from our camera:

ffmpeg -y -f vfwcap -r 25 -i 0 out.mp4

Where "-i 0" is the index (zero based) in the list of present capture devices ("Driver 0" in this instance).

Linux

On Linux, we can use ​video4linux2 (or shortly "v4l2") input device to capture live input (such as web camera), like this:

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

or

ffmpeg -f v4l2 -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 the fedora/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>