Card are useful?

For 5 cents each,if that's enough for you. its a good business for those who buy bundle scrap
A common card is worth around 10gems, dont bother
Gems can be sold in the market once you are able to create a bag (1000gems)
By checking for how much people are willing to pay. VN cards are usually more valuable
The Pro's of crafting badges

These questions have been answered with good detail long ago.

Most things you need to know of Steam cards.
What are Steam gems?

The most value you can get out of cards is selling them. They add monetary credit to your Steam wallet for future purchases (think of it like a small cash-back reward for buying a game). Most cards are cheap (less than $0.07) but some are highly sought after and thus worth a lot more. The market determines their value. To find out how much a card is currently selling for you can click on it in your inventory and use the box to the right to see details about the card and open up buy/sell screens.

Try to change for a nice background, or trade it for a game, once i saw a guy who wants 3 cards for 1 game

If you care about customizing your profile or if you don't have enough slots for your friendlist, then you might want to look into crafting badges. If not, just sell the cards. Gems arent worth it, just ignore them.

Cards and badges are really just collectibles that some of us are addicted to that devs and valve profit on.

you can even bu games,

0.30~ value in cards for a game bundled , check trades section

-5. You're eligible for random booster pack drops (unopened packs containing 3 cards) for a specific game after you get all the card drops you can for that game.These packs may drop at anytime and you're not required to be running the game.
Also, leveling up on Steam increases your booster pack drop rate (+20% every 10 levels or so), increases your friend limit (+5 friend slots per level) and your showcase limit (+1 every 10 levels)

I'm lost too, when it comes to these badges/gems/cards and what not.I play the games I like to play, and I don't care what my Steamlevel is or what is means. Let it be zero for all I care... :)

But then, CR7CAMIAO has a point, it might be worth something. Turning them stuff into new games?

If u get your steam profile to level 6969 u will unlock all steam games

Well, when we used to be kids, we collected marbles or pokemon cards or pogs or other things I don't remember. So it's similar with steam. You can collect badges and level up imaginary stuff. But that's all, it has no real use. It's up to you whether you want to collect stuff or not =). If you don't care about this, then just sell cards on market and earn some cents.
Oh just remembered.. when you level up you get more slots for new friends in your friendlist. Though I never reached the limit so I don't know what is the exact number. That is the only "useful" thing you get by leveling up, I know of.

Cards are useless. Money is useful. Cards can be exchanged for money. Money can be exchanged for goods and services.

They are useful to trade too, i bought a tons of games with cards.

Should i sell them? You can use them to trade with others for games or other cards, if you have no desire to trade them... selling them is a useful option You can also complete badges to level up your steam level, which has additional benefits, although depending upon what you want to do it might not be that useful.. Should at the very least get your level up to whatever one is necessary to get booster packs.. think it's level 5..

Are they worth turning them into gems? Absolutely not, unless you have cheapo cards that nobody will buy, they are more valuable to save and trade, or sell than turn into Gems..

What are gems? Gems were used during one of the sale events a few years ago, you could use them to bid on games that steam was giving away. Actually a huge glitch happened earlier on in the sale where people could get unlimited amounts of gems , which caused Sacks to sell for like .05 cent... obviously a major oversite on valve part, they pulled back all the gems, refunded all the money and restarted the event..

How do i know witch card is valuable and witch is not? You can use Item Value sorter site which will give you a run down on how valuable your cards are.. You can also shift through your inventory, clicking on a card will give you the value it's currently selling at..

Are there other stuff i need to know about?

If you are interested in badges, then make them, otherwise use something like Enhanced Steam to list them at a decent price and wait for them to sell. (Or instant sell if you need the money now!)

If you really want to get into it you can also make badges in hopes of getting good background and the like, which can sometimes sell for a fair bit. That said, most games their Backgrounds and emoticons are all but worthless and you're better off just selling the cards.

cards are money.

is money useful?

tl;dr As Mullins said: cards are money.

Long answer text wall:

Granted, I have alts, and of course get games from bundles and such, but my alts pretty much only redeem free games. I almost never buy something just to put on an alt for farming. I do not craft except during sale card drop periods (when you receive a card for each craft--this begins about two weeks before the sale, and during the sale you get up to three summer cards per day per account by exploring your discovery queue, which can be done with a script). Because of free games and alts I do not have to do much buying in the market in order to get full sets.

So, I crafted from level 65 to level 95 AND I sold the vast majority of summer sale cards (crafting only a level 7 badge) for about $30.00 of profit.

Crafting is completely optional and there is very little benefit for going to level 95 from level 65. I'd say the only level that matters at present is level 8 (the minimum level to be able to get cards from exploring your discovery queue during the big sales). If you want an extra showcase or two, or of you want to have hordes of people on your friends list, then crafting beyond 8 has obvious benefits (but diminishing returns).

Cards are free money. Free games are free money. Even if you just have your single Steam account. Though personally I don't like to clog my account up with utter shite so I avoid redeeming crappy free games to it.

Sell or craft? Up to you. If you are listing items at $0.03 or $0.04 when fees are gonna snatch away $0.02 of that, you're wasting your time.

What are gems? Steam has a FAQ to answer a question as basic as that one.

Craft into gems? No. Gems are best exploited by figuring out which games turn you a small net profit with the cost of the cards you can sell from buying a booster with gems vs. the cost of a sack of gems. AFAIK since the price of gems spiked recently this is not as awesome as it used to be, though maybe the market has corrected itself then. I don't know, I don't really deal with that.

How do you know which cards are valuable? By looking at their prices? If you want to level up by crafting you should be crafting the cheapest badges. A site like Steam Card Exchange can tell you values, but you still have to pay attention to how many cards you have as well.

There's lots of stuff to know but this comes with experience and research. The first thing you should do is put in the necessary effort to answer the simplest questions without relying on others to help you, such as what gems are.

Boosters aren't that much of a factor, you get boosters from the most "popular" games (which are often free games), and you get more boosters by having more eligible accounts than having a single account eligible for a wide variety of boosters.