When I type dpkg -l | grep xserver-xorg, I get a list of installed packages and their versions:
ii xserver-xorg 1:7.7+3~deb7u1
ii xserver-xorg-core 2:1.12.4-6
ii xserver-xorg-input-evdev 1:2.7.0-1+b1
ii xserver-xorg-input-kbd 1:1.6.1-1+b1
ii xserver-xorg-input-mouse 1:1.7.2-3
ii xserver-xorg-video-radeon 1:6.14.4-8
I am wondering what the number in front of the colon means, i.e. the number 2 in 2:1.12.4-6
The version numbers in Debian are of the form
[epoch:]upstream_version[-debian_revision]
where
epoch is a single (generally small) unsigned integer, which is included to allow mistakes in the version numbers of older versions of a package. If omitted, the epoch is assumed to be zero.
upstream_version is usually the version number of the original source package from which the .deb file has been made. It is usually kept the same as the format used for the upstream source.
debian_revision specifies the version of the Debian package based on the upstream version. It is optional and is omitted in cases where a piece of software was written specifically to be a Debian package.
The 2 is the epoch.
More fully, from man deb-version:
[epoch:]upstream-version[-debian-revision]
epoch
This is a single (generally small) unsigned integer. It may be omitted, in which
case zero is assumed. If it is omitted then the upstream-version may not contain
any colons.
It is provided to allow mistakes in the version numbers of older versions of a
package, and also a package's previous version numbering schemes, to be left
behind.
upstream-version
This is the main part of the version number. It is usually the version number of
the original ("upstream") package from which the .deb file has been made, if this
is applicable. Usually this will be in the same format as that specified by the
upstream author(s); however, it may need to be reformatted to fit into the package
management system's format and comparison scheme.
The comparison behavior of the package management system with respect to the
upstream-version is described below. The upstream-version portion of the version
number is mandatory.
The upstream-version may contain only alphanumerics ("A-Za-z0-9") and the
characters . + - : ~ (full stop, plus, hyphen, colon, tilde) and should start with
a digit. If there is no debian-revision then hyphens are not allowed; if there is
no epoch then colons are not allowed.
debian-revision
This part of the version number specifies the version of the Debian package based
on the upstream version. It may contain only alphanumerics and the characters + .
~ (plus, full stop, tilde) and is compared in the same way as the upstream-version
is.
It is optional; if it isn't present then the upstream-version may not contain a
hyphen. This format represents the case where a piece of software was written
specifically to be turned into a Debian package, and so there is only one
"debianisation" of it and therefore no revision indication is required.
It is conventional to restart the debian-revision at '1' each time time the
upstream-version is increased.
Dpkg will break the version number apart at the last hyphen in the string (if there
is one) to determine the upstream-version and debian-revision. The absence of a
debian-revision compares earlier than the presence of one (but note that the
debian-revision is the least significant part of the version number).