Every block contains a hash of the previous block. This has the effect of creating a chain of blocks from the genesis block to the current block. Each block is guaranteed to come after the previous block chronologically because the previous block's hash would otherwise not be known. The blocks thus form a history of the bitcoin state, with which address each bitcoin was assigned at a given time, it is not possible to change the contents of any block without invalidating the successive blocks (because its hash would change).
Each block contains a Proof-of-work that protects the block chain. Generators will build on the valid chain that contains the most work (blocks don't neccesarily contain the same amount of work).
For any block on the chain, there is only one path to the genesis block. Coming from the genesis block, however, there can be forks. One-block forks are created from time to time when two blocks are created just a few seconds apart. When that happens, generating nodes build onto whichever one of the blocks they received first. Whichever block ends up being included in the next block becomes part of the main chain because that chain is longer. More serious forks have occurred after fixing bugs that required backward-incompatible changes.
Blocks in shorter chains (or invalid chains) are called "orphan blocks", and while they are stored, they are not used for anything. When a block becomes an orphan block, all of its valid transactions are re-added to the pool of queued transactions and will be included in another block. The 50 BTC reward for the orphan block will be lost, which is why a network-enforced 100-block maturation time for generations exists.
Because a block can only reference one previous block, it is impossible for two forked chains to merge.