The word blockchain is defined with combining two words: “block” and “chain”. As a matter of fact, this technology is based on a chain of blocks. The blockchain is a system to write and register the data. The difference between this type of technology and other systems is that the registered information on it can be shared and accessible by all network users which is secured by a mathematical mechanism called “cryptography”.
So it is almost impossible to change the written data on the blocks so the data will be immutable forever. Bitcoin was the first application of this technology and used to store users’ assets or money. If you assume blockchain as an OS, Bitcoin is a software app on it.