Bitcoin core was developed and distributed by Satoshi Nagamoto as the official wallet client software.
Initially it was changed to Bitcoin and Bitcoin-QT, and now it's changed to Bitcoin Core.
A block chain of more than 100 GB must be downloaded in order to be able to use a bit coin core, meaning that it is a full client used by bit coin nodes to create a bit coin network.
With changes in bit coin, developers change the underlying bit coin protocol and maintain the ibsecp256k1 cryptographic library.
Because of this problem, software other than bit coin core is being developed and used.
The MIT Digital Currency Initiative invests some of its development.