Credit Cards typically charge interest on money you borrow from them. They work in one of two ways.
Most cards will not charge you any interest if you pay the balance in full each month. You typically have around 25 days (the "grace period") to pay that off. If that's the case, then you will use your credit card without any cost to yourself.
However, if you do not pay it in full by that point, then you will owe 19.9% interest on the balance, typically from the day you charged the payment (so, retroactively). You'll also immediately begin owing interest on anything else you charge - typically, even if you do then pay the next month the entire balance on time. It's typically a "daily" rate, which means that the annual rate (APR) is divided into its daily rate (think the APR divided by 365 - though it's a bit different than that, since it's the rate which would be 19.9% annualized when you realize interest is paid on interest).
Say in your case it's 0.05% daily - that means, each day, 0.05% is added to your balance due. If you charged $1000 on day one and never made a payment (but never had to - ignore penalties here), you'd owe $1199 at the end of the year, paying $199 interest (19.9*1000).
Note that your interest is calculated on the daily balance, not on your actual credit limit - if you only charge $100, you'd owe $19.90 interest, not $199.
Also note that this simplifies what they're actually doing. They often use things like "average daily balance" calculations and such to work out actual interest charged; they tend to be similar to what I'm describing, but usually favor the bank a bit (or, are simpler to calculate).
Finally: some credit cards do not have a grace period. In the US, most do, but not all; in other countries it may be less common. Some simply charge you interest from day one.
As far as "Standard Purchases", that means buying services or goods. Using your credit card for cash advances (i.e., receiving cash from an ATM), using those checks they mail you, or for cash-like purchases (for example, at a casino), are often under a different scheme; they may have the same rate, or a different rate. They likely incur interest from the moment cash is produced (no grace period), and they may involve additional fees. Never use cash advances unless you absolutely cannot avoid it.
