Indeed, blocks contain the hash of previous block, forming a linked list or chain, thus
the blockchain. (tadaaaa !)
However the hash of a previous block can be contained by more than one candidate sibling at a given time, this is fairly common. This could be caused for example by two miners who solved a block almost at the same time and broadcasted their block, or by a double spend attempt. Since the network is distributed there is no way to know what is the "right" block.
The majority of the network picks a block they want to extend (for example, the block that does not contain what they ascertain to be a double spend) and they continue to extend that subchain. The algorithm is designed as to use the longest chain as the "main" chain, however it's fairly common that the majority of the network has a different view on which is the main chain. When this happens a reorganization takes place, and the miners who were working on extending the alternate subchain switch to the correct one.
https://en.bitcoin.it/wiki/Chain_ReorganizationThe hash power used to create the orphan subchain is wasted and no coins are credited for it (technically there are, but you can spend them only in the orphan chain which nobody accepts). A fast block rate (such as in litecoin or feathercoin) favors block reorgs, and it's an open debate on what is the best security/speed tradeoff.