How to send and receive cryptocurrencies
A Public Key is a code of numbers and letters that is unique and discribes your "wallet". This code is visible for everybody on the transaction history. Now, you can think of it as your current address. Everybody can create new "adresses"which gives them a public key. This key is important when you want to receive certain cryptocurrencies. So in case someone wants to send you coins, he needs to know this adress.
The private key is another code, that verifies that you are the true owner of the wallet. This code is the ultimate access to your cryptocurrencies. It is the password, to open what is stored in the wallet. You need this private key in order to access your cryptocurrencies and send them to someone else.
It is very important to know, that every wallet and every cryptocurrency can have their own blockchain they work on. So you have to be aware which coins you send where to. Because these different cryptocurrency systems aren't all the same, sending cryptocurrencies to a wrong adress can cause a loss of all the cryptocurrencies. For example, if you send Bitcoin to an ETHEREUM public key (address), you risk loosing your Bitcoin for ever.
How does a transaction work?
In case you wonder, how such a transaction works, we try to explain it very short.
If you create a transaction, your public key creates information, that goes into a public so called memory pool. There, the transaction waits to be verified through miners and put on a new block. Once this new block is added to the blockchain, the transaction is verified and valid. But what information, you might ask?
When sending a transaction, you need to verify it's you. In order to do so, you need to create a digital signature to the transaction. By putting in your private key, an algorithm automatically creates a so called hash, a random, unique code. Your public key (wallet address) verifies that the hash actually was created by you. In other words: The hash verifies it is you, without you having to expose your private key. This hash can be verified by anyone on the blockchain.