Credit cards are something that a user is more likely to come across than investments. I remember someone had questions about them a while back on c/main.
APR - Yearly interest on money borrowed. It varies depending on your existing credit (or lack thereof) as well as what the credit company is offering. Usually when you're just starting out you'll get high APRs. This means when you borrow, there's high interest. That doesn't mean you shouldn't use the cards with high interest, just that you should pay it back quickly and be very careful what you borrow on them. Eventually you may get an offer with 0% APR for a limited time (several months or a year). It's important to note that cards will have variable APR meaning if you're late then the interest rate raises. This is part of why CCs can be bad.
Credit limit - How much you can borrow in total. Borrowing up to this limit can influence your credit score. You don't want to constantly be using a high percentage of your credit limit. CC companies will notice and probably offer to up your limit or send you new cards. This is predatory and you should watch out for it.
Annual Fee - Some cards require you to pay to use them. Not unlike banking fees. Try to avoid these cards if you can. You might have to deal with a few of them while building credit but at some point, but eventually you should get good enough offers that there will be no annual fees.
Credit Score - This isn't only tied to your credit cards, but anything you borrow. Using too much of your credit can cause it to go down. Getting a lot of cards at once can cause it to go down. If you don't use some cards, the CC company will close the account. This could also ding your credit score.
Simple strategy for building credit with CCs: Use them like a debit card but don't spend your cash. Put groceries and gas on a card, pay it off when the bill comes due. Rotate cards as you get more. Don't do any big borrowing unless it's an emergency. It will be tempting to use it on a big purchase, but don't. Once you borrow, if you can't pay it back immediately, the interest starts. And even with the interest free cards, you don't want to collect too much debt and then have an emergency put you behind. By paying off your cards immediately the CC company gets no interest. And if they get no fees either, then you win. You're a deadbeat because you pay off your stuff and they can't weasel any money out of you.
I don't know much about what to do if you have to max out a card for an emergency like dental surgery or car repair. I'd be curious about some strategies there if anyone has them.
Also, Obummer did sign a bill that made it so after 7 years, lenders have to drop the debt. But they will still hassle you the whole time. It's a tiny silver lining because at least it's not debtors prison and they do have to stop calling you eventually.
Here's something regarding credit cards/credit lines that may help some people and that they may not be aware of. In fact, I only became aware of the possibility a few months ago. Disclaimer: it does require a 401k and it helps when your company or organization aids in funding it, even if only a little bit.
It's fairly easy to get low(ish) levels of credit on one card or account, something in the $1k-2k range, but even a few hundred will work just fine. I'm going to use my particular circumstance as an example, but this obviously isn't the only way to do it.
I was able to get approved for $1.9k through paypal credit last summer even with a modest credit score. Paypal credit allows you to defer interest (which is an absurd 24%) for the first 6 months on every purchase over $100 (this is not unique to paypal credit, a lot of credit line have similar 'deals' that will be offered). This of course means that at 6 months + 1day you owe that much interest on the purchase. However, one of the advantages of having a 401k is that you can loan yourself money from it (a maximum of around 1/2 of the total amount) at very low interest rates for up to (I think) 5 years. I'm talking 4-6% which is insanely better than the 24% on my paypal credit and most other credit cards. And here's the kicker: the interest you pay back on your 401k goes right back into your 401k. That's right, you're not paying that interest to the bank, you're paying it back to yourself. Now, you have to be careful because if you lose your job or otherwise lose the 401k you're on the hook for the money borrowed immediately; but, then again, you get to set the terms of how long you want to loan that money to yourself from between 1-5 years.
This is great because it's easy for me to make the minimum payments for the first six months, it's something in the neighborhood of $40. And then, toward the end of six months, I loan myself whatever is left on the paypal balance from my 401k and pay myself back to the tune of $40ish dollars a month, only this time it's at 4% interest instead of 24%. And, like I said, that interest I'm paying goes right back into my account instead of disappearing to the bank.