Ok, maybe this is a noob question, however...
I needed to install some packets from extra-testing and extra-devel catalogs, so I added them.
I can see that, for many applications, for example macchanger here:
http://maemo.org/packages/view/macchanger/
there are Extra, Extra-testing and Extra-devel versions, all 1.5.0-4.
So, if I have all the catalogs, which version will be installed by default?
To avoid any trouble, I deactivated extra-testing and extra-devel catalogs after installing the packages I needed.

By the way, if for example I want to install a package from terminal which belongs to extra-devel catalog, by I don't have activated that catalog, it won't install, right?
For example, I tried to install John The Ripper with apt-get install john, but it installed only after adding extra-devel catalog.

If the Versions are the same it got promoted from devel to extras-testing and then to extras.
Always the newest version will be installed, which is devel if activated, or respectively testing.

To avoid any trouble you can activate testing and devel and then deactivate automatic updates to testing/devel.
http://wiki.maemo.org/Extras-devel#Limiting_the_damage

Thanks! Now it's more clear!

Another tip is to install Faster Application Manager or better use apt-get.
http://maemo.org/downloads/product/Maemo5/fapman/
Because with activated testing/devel the default Application Manager gets terrible slow.

Eh, I noticed that!
I also notice that sometimes, when I enable/disable a catalog, it says: no applications found, and I have to reboot the phone.