A fork is said to have happened when there is a conflict among the nodes regarding the validity of a blockchain, that is, more than one blockchain happens to be in the network, and every blockchain is validated for some miners. There are three kinds of forks: regular forks, soft fork, and hard fork.
A regular fork is a temporary conflict occurring due to two or more miners finding a block at nearly the same time. It's resolved when one of them has more difficulty than the other.
A change to the source code could cause conflicts. Depending on the type of conflict, it may require miners with more than 50% of hash power to upgrade or all miners to upgrade to resolve the conflict. When it requires miners with more than 50% of hash power to upgrade to resolve the conflict, its called a soft fork, whereas when it requires all the miners to upgrade to resolve the conflict, its called a hard fork. An example of a soft fork would be if update to the source code invalidates subset of old blocks...