The following is my own ranting and raving. For another (similar) view check out Wordman's lengthy discussion on the subject. Credsticks are also discussed in the NAGRL, but who wants to read that. There is also a lengthy and in-depth discussion on credsticks here.
Credsticks are standardized world-wide, at least in the markets sharing the Nuyen as a common currency. They are issued by whatever bank the character happens to be dealing with and usually contain at least some af a character's SIN information.
Credsticks are about 6cm in length with a width tapering from .75cm to .5cm The credstick itself does not contain any sort of authentication (such as thumbprint scanner). That function is integral in whatever hardware is used to facilitate a transfer of funds or just access it.
Any currency other than Nuyen, such as corporate script, is usually maintained on something similar to a credstick, but unique to that currency. These consist of cards (similar to a credit card of today), rings, large coin-shaped cards, etc. The physical representation of these can be quite diverse. Transfer between credsticks of different currency must use a piece of hardware that can facilitate the transfer with up-to-the-second exchange rates. This means that a Nuyen credstick cannot transfer directly to a Renraku-script stick.
Credsticks with SIN only capacity can be purchased from many licensed agencies. These include Government offices, corporate personal offices and the like. These sticks usually have no currency capacity and are used as an ID card only.
In Germany no credsticks are used. Their equivalent is the EBBY or
"Ebie" - sized like a pack of cigarets(sp). Also in Germany there
are 3 different currencies: NuYen (used mainly by the big corps), EC (the
standard currency) and the old DM (only aviable in coins (plastic with a
microprocessor inside, sorta like certified cred. They are colour-coded
and come in up to 500 DM sizes, though they are seldom. Many people will
not accept NuYen and a lot of people - especially in smaller towns or
lower lifestyle - won't take anything but DM.
What's in a SIN
These are some of the things that would most likely be stored on your credstick. The SIN (System Identification Number) itself is just an encrypted and indexed jumble of a Social Security-like number, your name, date of birth and a bunch of other crap all stuffed in and fed to an algorithm. Cryptic, neh?
Basically these little jobbies are PDAs (Personal Data Assistants) with a phone and pretty good data capacity built into it. These are much more useful than most people think. You can transfer cred over the phone to the person on the other end, or to your bank account on another continent (without the hassle of talking to a teller).
Useful things you can do with your pocket secretary
This is as good a place as any to address this issue. Messaging in the world of Shadowrun has become almost a single thing. Phone calls can still be made directly to a person, but there are more options for just how this can be done:
Remeber, a large portion of the street population is only "Iconoliterate", and unable to read written text. Many corp individuals may be unaware of this, but those that are can use it to their advantage.