The German Post Deutsche Post World Net is very efficient: you can count on receiving your mail the day after it was sent from anywhere inside Germany (95% of the mail in 1 day, 99% inside 2 days). So much so, that some merchants expect to be paid one or two days after they send you the bills! There is pick-up and delivery 6 days a week, including Saturday which is considered a working day. It is also maybe the oldest international postal service, as it was created in the Middle Ages.
But the German Post is more than your ordinary postal service: it also offers banking services. You can open an account there as you would in any other bank. The main difference is that their fees are usually less expensive than in other banks. You can also deposit and withdraw money from your account at any post office. They also now offer mortgages, credit cards and even insurance!
It costs, at the moment (2008), 1.00 euro to send a postcard overseas and 1.70 euros for a standard size letter. The rates are 0.45 for a postcard and 0.55 for a letter delivered within Germany, and 0.65 for a postcard and 0.70 for a letter within Europe. Prices are subject to change, better ask your post office for more info.
You can also buy stamps directly at the automatic stamp dispensing machines (Deutsche Briefmarken machine). A stamp is called Briefmarke in German, a letter is a Brief, an envelop is an Umschlag, a mail box is a Briefkasten, a post office box (PO box) is a Postfach and a Postcard is...a Postcard!
Please note: In Europe, there is a different "standard" size of paper that is typically used for letters and non-legal documents - i.e. the "A4"-size, which is slightly different from a USA standard size letter for the simple reason that, like most of the world, Europeans use the Metric system NOT the Imperial system. So, the standard size of paper for European letters is: 21 X 29.7 cm, while the Imperial letter is: 8 ½ X 11 inches or about 21.5 X 28 cm.
In other words, the usual European letter is a bit narrower and longer than the USA one. And when it is time to send a letter by mail, you might find that your USA size letter feels a bit tight in its European envelope (22 cm wide instead of 24.2 for USA envelopes). But it should still fit. And in any case, German Post will accept your envelope even if theirs are slightly wider (but just so).
By the way, if you bring your own printer, you might find that the German paper does not exactly align with your word-processor settings and the same is true for USA paper going into German printers. Once again it is because of the two measurement systems. But we have been using our paper from North America on a German printer for some time now, and after a bit of adjustment to the margins, it works just fine.
The mail box, Briefkasten, is yellow and displays the sign of the Deutsche Post (an old-fashioned hunting horn called the Post Horn). The deposit slot is sometimes on the side of the box instead of on the front.
Deutsche Post still delivers right to your door in most areas of the country. There are also thousands of German post service points across the country and, like other stores, they close during lunch hour and from Saturday afternoon (13h30 or 14h00) until Monday morning. In some regions, they might also be closed on Wednesday afternoons. Also, depending on the region, the postmen now sell stamps, pick up packages, accept bank deposits, cash cheques and can even sell you a phone card!
Another little difference with North America, U.K. and Australia is the way to write the address on the envelope. In Germany you must first write the name of the street then the house number. You must also write the postal code before the city's name. For example: HauptStrasse 28, 69141 Heidelberg, Deustchland.
Make sure everyone who wants to send you mail knows about this system, otherwise your mail might end up somewhere else...