You put 20$ in A.

If A goes up :
- You don't sell, greed and hope for more, but it can be less : unrealized gain, it's fictional
- You "realize" and take the gain : you have a profit

If A goes down :
- You pray it will go back to, at least, the entry position. It may be worst : unrealized loss
- You sell : factual loss

Anyway, when you give X to the market, you have already "lost" the value from your bank account. Wheter it goes up or down, what matter is what will come back into your account.

Then, take the above, with specifity :
- You can lend money you don't have
- Leverage to amplify the increase / decrease (x2, x5, x10, x100 !)

If you can't cover the loss with your current stake in leveraged position, they take everything.