Where to find help:

1.) See the text file in aptitude itself:
 ctrl-T -> Help -> Handbook

It will directly open /usr/share/aptitude/README

2.) Or install aptitude-doc-en (or your language) and browse:
 /usr/share/doc/aptitude/html/en/index.html


Especially interesting is the table
file:///usr/share/doc/aptitude/html/en/ch02s04s05.html



Example: lets search for c++ http libraries building on
top of boost:

aptitude search "?or(?section(libs),?tag(role::shared-lib)) ?description(HTTP) ?or(?depends(libstdc\+\+6),?tag(c\+\+)) ?depends(libboost-system1.49.0)"


It uses following tricks:
In many packages the tags are not correctly filled out.
So to either use shared-lib (which would be the correct search phrase),
but also everything below the section libs, you can use:
?or(?section(libs),?tag(role::shared-lib))

The same trick is used for the c++ tag (or more specifically implemented-in::c\+\+):
?or(?depends(libstdc\+\+6),?tag(c\+\+))

Then it is of course always useful to reduce the results by searching
within description or package name. (At least as long as tags are used
so sparse).


Example
aptitude seach "?maintainer(Marillat)"