Windows

On Windows, we can use vfwcap input device to capture live input (such as web camera).

First, we'll try to list the supported capture devices, connected to our 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

Now, we can try to grab something from our camera, with something like this:

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").

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>