^^ It is definitely possible to buy simple things like gas and pay it off immediately. I think there is a daily interest accrual, not that it's going to be more than pennies if you have a really low balance. I've been through the credit gauntlet, ended in bankruptcy after quite a lot of years and a tremendous ending amount of debt (due to opiate addiction and a non-working/non-functional spouse). That was a year and a half ago, just recently I got an offer for an unsecured credit card (with secured credit cards you have to put a deposit down up front for the entire balance), and got it, so I can start rebuilding my credit. What I do is, once or twice a month, I buy gas or groceries with the card and then pay it off in full right when I get home. That way I don't incur any interest charges and I have a bunch of payments posted that reflect "paid in full". Actually I think that as long as you don't go into a new billing cycle with a balance, you won't get charged any interest if you pay it off in full on time by the end of the billing cycle.
You get some manner of negative marks if your balance goes over 50%, and then again over I think it's 80% (and maxed out is worse). It's really best to use them sparingly and pay it all off each month, and only use it for something more in an emergency (in which case they're nice to have). I agree it's a smart idea to build your credit up in this society, most notably because if you ever want to buy a house your interest rate will be lower the higher your credit score is, and that makes a tremendous difference with a mortgage because it's such a large loan amount, even half a percentage point will lower your monthly payment and the total amount you'll spend in the life of the loan. Or a car loan, or you never know, someday you might need a loan of a different sort and it's much better for you when you have a high credit score (not to mention you'll be sure to not get refused). I think it's weird that they charge people who show evidence they're less able to handle it a higher interest rate, but that's how it goes.
APR means annual percentage rate... though most companies charge interest daily or monthly (usually they just add on a charge of the amount of interest accrued in a month, at the end of the month in one chunk), it's calculated from the APR. If you let a balance sit for a whole year, that APR is the percentage of that balance that will be charged in interest. If you had a $100 balance, and a 20% APR, you'd be charged $20 in a year, but just 1/12 of that each month.
