Can anyone tell me what Blockchain is?

BLOCKCHAIN is a method used to exchange information, usually cypted and protected,
without being dependent on any government or any institution.
The amazing thing is that everythig is recorded and cannot be changed or altered.
The development of the blockchain gives birth to cryptocurrencies.
These digital tokens that allow payments directly through an online system without any bank!
The first time I asked some people who got different wallets and know many things
about cryptocurrences, I was flabbergasted!
My quesion was that simple: how many cryptocurrencies exist? I was waiting as an answer for something like 10 or maybe 20 cryptocurrencies but when I was told there is more that 6000!!!!
I really was empressed by the high number of these digital currencies!
In the next articles I will talk about the most well known crypto currencies and any potenial of inestment on these currencies.