Question

Could somebody explain membrane fusion of viruses into a host cell?

Answer 1

Viral receptors attach to the receptors on the surface of the membrane of the host cell, which starts the fusion of the two membranes, as the viral envelope unfolds. The viral envelope pretty much merges together with the host cell's membrane, and releases the viral content into the host cell.
Here's a good picture to help you visualize this...

Answer 2

What's hemifusion?

Answer 3

Well, the prefix -hemi means half or partial, so hemifusion is just a small part of the whole process of fusion.