'deb' is not a command. It is used in sources.list file to indicate a Debian software repository.
From Ubuntu Manpage - sources.list:
The source list is designed to support any number of active sources
and a variety of source media. The file lists one source per line,
with the most preferred source listed first. The format of each line
is: type uri args. The first item, type determines the format for
args. uri is a Universal Resource Identifier (URI), which is a
superset of the more specific and well-known Universal Resource
Locator, or URL.
The deb type describes a typical two-level Debian archive,
distribution/component. The format for a sources.list entry using the
deb and deb-src types is:
deb [ options ] uri distribution [component1] [component2] [...]
The URI for the deb type must specify the base of the Debian
distribution, from which APT will find the information it needs.
distribution can specify an exact path, in which case the components
must be omitted and distribution must end with a slash (/). This is
useful for when the case only a particular sub-section of the archive
denoted by the URI is of interest. If distribution does not specify an
exact path, at least one component must be present.
So, if I have deb http://archive.ubuntu.com/ubuntu/ quantal main restricted in sources.list it says I have a Debian archive which is based on "http://archive.ubuntu.com/ubuntu/", the distribution is "quantal" and the components are "main" and "restricted".
