mjhjmtu

I have installed some packages but they obviously don't work on my system.

Can anyone explain what A beside i means?

i A libstrongswan - strongSwan utility and crypto library 
p libstrongswan:i386 - strongSwan utility and crypto library 
p libstrongswan-extra-plugins - strongSwan utility and crypto library (extra pl
p libstrongswan-extra-plugins:i386 - strongSwan utility and crypto library (extra pl
i A libstrongswan-standard-plugins - strongSwan utility and crypto library (standard
p libstrongswan-standard-plugins:i386 - strongSwan utility and crypto library (standard
p network-manager-strongswan - Grundgerüst zur Netzwerkverwaltung – strongSwan
p network-manager-strongswan:i386 - Grundgerüst zur Netzwerkverwaltung – strongSwan
i strongswan - IPsec VPN solution metapackage 
i A strongswan-charon - strongSwan Internet Key Exchange daemon 
p strongswan-charon:i386 - strongSwan Internet Key Exchange daemon 
p strongswan-dbg - strongSwan library and binaries - debugging sym
p strongswan-dbg:i386 - strongSwan library and binaries - debugging sym
p strongswan-ike - strongSwan Internet Key Exchange daemon (transi
p strongswan-ikev1 - strongSwan IKEv1 daemon, transitional package 
p strongswan-ikev2 - strongSwan IKEv2 daemon, transitional package 
i A strongswan-libcharon - strongSwan charon library 

The first character of each line indicates the current state of the package. i means the package is installed, and p means that no trace of the package exists on the system. The second character indicates the stored action (if any, otherwise a blank space is displayed) to be performed on the package. If the third character is A, the package was automatically installed.

From the results of man aptitude:

search
 Searches for packages matching one of the patterns supplied on the
 command line. All packages which match any of the given patterns
 will be displayed; for instance, "aptitude search '~N' edit" will
 list all "new" packages and all packages whose name contains
 "edit". For more information on search patterns, see the section
 "Search Patterns" in the aptitude reference manual.

 Unless you pass the -F option, the output of aptitude search will 
 look something like this: 

 i apt - Advanced front-end for dpkg
 pi apt-build - frontend to apt to build and install architecture optimized packages
 cp apt-file - APT package searching utility -- command-
 ihA raptor-utils - Raptor RDF Parser utilities

 Each search result is listed on a separate line. The first
 character of each line indicates the current state of the package:
 the most common states are p, meaning that no trace of the package
 exists on the system, c, meaning that the package was deleted but
 its configuration files remain on the system, i, meaning that the
 package is installed, and v, meaning that the package is virtual.
 The second character indicates the stored action (if any; otherwise
 a blank space is displayed) to be performed on the package, with
 the most common actions being i, meaning that the package will be
 installed, d, meaning that the package will be deleted, and p,
 meaning that the package and its configuration files will be
 removed. If the third character is A, the package was automatically
 installed.

 For a complete list of the possible state and action flags, see the
 section "Accessing Package Information" in the aptitude reference
 guide. To customize the output of search, see the command-line
 options -F and --sort.

See man aptitude or its embedded help:

If the third character is A, the package was automatically installed.

It means the package has been installed to satisfy dependencies of other packages and it would be removed if the depending package(s) were removed.

To check which package depends on an automatically installed package,
use the command aptitude why <package> or (repeatedly) press I in the Aptitude TUI.