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man apt-cache has this to say on the matter:

showpkg displays information about the packages listed on the command line. Remaining arguments are package names. The available versions and reverse dependencies of each package listed are listed, as well as forward dependencies for each version. Forward (normal) dependencies are those packages upon which the package in question depends; reverse dependencies are those packages that depend upon the package in question. Thus, forward dependencies must be satisfied for a package, but reverse dependencies need not be. For instance, apt-cache showpkg libreadline2 would produce output similar to the following:

Package: libreadline2
Versions: 2.1-12(/var/state/apt/lists/foo_Packages),
Reverse Depends:
2.1-12 - libc5 (2 5.4.0-0) ncurses3.0 (0 (null))
2.1-12 -
Reverse Provides:

Thus it may be seen that libreadline2, version 2.1-12, depends on libc5 and ncurses3.0 which must be installed for libreadline2 to work. In turn, libreadlineg2 and libreadline2-altdev depend on libreadline2. If libreadline2 is installed, libc5 and ncurses3.0 (and ldso) must also be installed; libreadlineg2 and libreadline2-altdev do not have to be installed. For the specific meaning of the remainder of the output it is best to consult the apt source code.

I thought maybe someone here has done that and I don't have to.

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up vote 3 down vote accepted

The provide field tells the package manager about a specific capability of the package in case there are alternatives available.

As an example, many packages require an MTA, but they don't care about which one. Thus, they have a dependency for the meta-package mail-transport-agent, which is provided by ie. exim4-daemon-light or nullmailer.

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+1. or postfix or sendmail or courier-mta or several others. – cas Sep 14 '11 at 10:44
how do i get a list of packages that provide something? – Janus Troelsen Feb 28 '12 at 17:21
@Ysangkok: Please post a proper question for this. – Sven Feb 28 '12 at 17:23

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