Bitcoin is the most known cryptocurrency. It is also the oldest and most tested one out there. Its main purpose is, to be a storage of value, albeit the community behind it is still in debate whether to scale it or not. Bitcoin is based on PoW (Proof of Work). In order to make transactions, a miner has to take the information of the transaction and put it on the next block that is going to be added on the blockchain. Because there are many transactions, it takes a while until the transaction goes through. This causes a lot of work for the miners, which gives them a reward in Bitcoin. The system is based on the value of the work that is put into the validation of these transactions, as well as all the different nodes supporting the system and the valid blockchain. 
Because the blockchain of Bitcoin is already very long, it takes a lot of space and work to synchronize all nodes with the recently made transactions. This makes the network slow, but also very secure.