Half-Life is about a guy, Gordon Freeman. He went to Black Mesa and worked at the test chamber, something went wrong, aliens started comming and trying to destroy Black Mesa. Your job is to go to the surface and meet up with Milatary Forces. But the government ordered the Milatary to kill the aliens, but also the workers of Black Mesa! Your next job is to go to Lamba Complex and telaport to Xen to destroy the big boss!
Fight all the aliens like Headcrabs, Vortigaunts, Zombies, Soldiers, and more enemies you will face, get weapons to defend yourself and save Black Mesa from the Xen Wildlife. This is Half-Life!
Headcrabs are from Half-Life, but this is a Half-Life 2 Headcrab. Headcrabs are 4 leged creatures that hop on Citizen's head and turn them into a Zombie. Headcrabs had some upgrades, they can barry themselfs in sand or dirt and pop out and hop on your head (really for you they just scratch), the Combine had taken Headcrabs from Xen and sent them on rockets and launch them in non-combine areas such as the small town of Ravenholm.
To destroy a Headcrab, just use your crowbar, it's the easiest way. If you don't have the crowbar when you fight a Headcrab (like in HL2: Episode One), use a pistol, SMG, or shotgun. Headcrabs can be tricky to kill, also 3 shots from your Gravity Gun can kill one in Half-Life 2.