Whilst they offer some great free services (edit: Google Apps is now only offered on paid subscriptions, even under the previous 5 user cap. But if you got in beforehand, you do get to keep it for free...), Google's products with their user-friendly interfaces can occaisonally be confusing for those of us who know what we're doing - a whole new set of terminology to learn just to be able to do what you had previously learned in the 'normal' / 'homebrew' / whatever way.
This is a quick cheatsheet on how to achieve some of the fundamental and more advanced mail behaviours and operations that you'd expect from any mailserver via your Google Apps admin console.
Mailboxes
Each mailbox in a Google Apps (let's just call it gapps from now on) domain corresponds to a domain user. If the users have their email turned on then they have an email address @ your domain, that's about it here. Since gapps is billed on a per-user basis, you will want to avoid making as many of these as possible - i.e. only when you have a need for a real inbox.
Aliases
Mail aliases are referred to as "nicknames" in Google Apps. You can add and administer these under the user settings page for the user you wish to manage. Each user can be aliased to different addresses not only on their domain but on any domain your company owns.
Forwarders
To create forwarding addresses (which do not store any recieved email themselves), you use the Groups feature in gapps to add organisational groups. Create a group with the email address desired, then add as many email addresses to that group as you wish. Specify the group as public and allow anyone on the internet to post to it when creating, and you're done. You can add email addresses from outside your domain as well, which is great for setting up rol-based addresses (eg. [email protected]) for forwarding on to the existing accounts of temporary or casual employees.
Forwarding & retaining email
Retention of messages gets a bit more complicated when you want to deliver them somewhere else. Email is only retained for 'real' user accounts, so this must be configured by mail forwarding at a GMail account level. Create a new user with the intended address and then login that user's gmail. To setup single-destination forwarding, go to Settings > Forwarding and POP/IMAP. Add the address you wish to forward to, verify it and set to forward all mail. That's it!
To forward to multiple addresses, first add and verify all forwarding addresses in the forwarding tab, then go to Filters, create a new filter matching To: your email address and select to forward to the desired email address in the next step. Note that you can use a | to denote multiple addresses, which is useful when the user has multiple mail aliases which you want to forward on.
After setting up forwarding to another GMail address, you will also need to sign in to the recieving email account, go to Settings > Accounts and click 'Add another email address you own'. The "Treat as an alias" setting is important - when checked, GMail will not automatically select your From: address when replying to emails sent to that aliased address, and messages you send to this address will come back to you. Disable to automatically choose the From: address as appropriate and only show emails sent to the other account in your sent folder.
Again - if you're forwarding all mail for an account and have no need to retain the messages, then you don't need to waste a user account - use a Google organisational group instead.