A block is “valid” if it obeys all of the Protocol rules; otherwise it is “invalid”. Clients will ignore invalid blocks when determining which chain is difficulty-wise-longest. Mining clients will not build on top of invalid blocks. The blockchain that represents the entire history of all transactions in the Bitcoin network is linear; there can be no [permanent] branches. If two miners solve a block at about the same time, a branch occurs, and miners may begin building on either of the two. The blockchain which grows faster becomes the “valid” blockchain for the network and the blocks in the losing chain become invalid. Thus there is a 100-block maturation time before block rewards and transaction fees may be spent. It acts as insurance against those BTC becoming invalid in the meantime.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc