A credstick is an antiquated way of carrying money, but still widely in use. Like paper money is today. You can connect a credstick to any standard com (think USB port) and transfers money from or certified cred to it. Certified cred has actually replaced credsticks per se. The certified cred is like an electronic bank draft, drawn against and issued by a bank. To validate its legitimacy, you simply need a matrix connection that will sync to the bank and validate it as the authoritative copy of the cred. Once you validate to the bank, it changes validation keys thereby invalidating any fraudulent copies... So it is in your best interest to validate certified cred as soon as you get it.

You can also get wireless credsticks which slave to a com and routinely validate the funds. Since that kinda defeats the purpose, they are not very popular in my Shadowrun. Most certified cred are drawn against either Cayman banks or Zurich Orbital, both being outside the legal reach of most governments meaning you have little concern of being traced. Though if you are concerned about the Big-10, you better have your cred drawn against the latter.

When you 'spend' certified cred (as opposed to simply transferring it), the bank it is drawn against simply makes a transfer to a legitimate account of funds. Legitimate businesses who receive large amounts of certified cred, tend to have that cred "washed" by transferring it through shell accounts. Illegitimate businesses simply transfer the certified cred around. Banks that deal in certified cred deal in a bit of speculation regarding percentages of certified cred that will never be redeemed.