If you go to Wifi in Settings.app and log in to a Wifi Network it will be remembered, the next time you go to that Network it will automaticaly connect. Aircrack is a "pack" off apps which work together to crack a network. They managed to make one app of that pack, the app that crack's the files that are "sniffed". So you first need an app to sniff packages and then you can crack it using Aircrack.
Also there is a app like Settings.app / Wifi that is called Stumbler, it works the same but it shows more Networks and shows more info about them. Stumbler cant connect to any networks it just shows them (you can connect using Settings.app).
Michiel
P.S.: Aircrack site:
http://www.aircrack-ng.org/doku.php