What's the difference between those coins? Does it have the same prices? Though I only hear about bitcoin a lot. What's altcoin, litcoin and dogecoin got to do?
They are different currencies. They have a different supply/demand and trade volume, that's why the price is different.
It's similar to why US dollar is worth more than Congo dollar
If you are creating a currency, at some point you have to choose how many units you will produce.
Will there be 1000 coins? 100.000 coins? Will you keep creating coins forever?
This is the coin limit. Bitcoin has a maximum of 21 million coins. Litecoin has a maximum of 84 million. Every coin has its coin limit.