I hope you are asking this question in reference to DPoS consensus protocol, keeping that in mind, I am answering.
Those who create and sign new blocks are called as block producers. Block producers are limited in numbers and are elected by voting process.
In steemit, the number of block producers are 21
In Bitshares, the number of block producers are 101.
Block validators are those who verify the blocks produced by block producers. They follow the consensus rule. Any user running the node can become a block validator and can verify the network.
You might have have seen in the witness of page of steemit, there are top 21 witness and also there so many witnesses having their respective rank.
So the top 21 witness are the block producers where as anyone from any rank in the witness page can be a block validators if they are running an active node.