A concentric diversification strategy allows a firm to produce similar products to an already established business. Let us say that a computer company, producing computers using towers, now starts to produce laptops.
Horizontal diversification allow a firm to start exploring other zones in terms of product manufacturing. If a company that used to produce television, now starts producing refrigerators, dryers etc. it's using horizontal diversification.
As for the conglomerate diversification strategies, this is where companies will look to enter a previously untapped market. So they want to move to a new industry.