Ivermectin and Ivomec are essentially the same thing. Ivomec is a brand name for a veterinary formulation of ivermectin, which is an anthelmintic (anti-parasitic) medication.
Ivermectin is also used in humans to treat certain parasitic infections, such as onchocerciasis (river blindness) and strongyloidiasis (threadworm). It has gained recent attention as a potential treatment for COVID-19, although its effectiveness for this purpose is still being studied and is not yet fully established.
So to summarize, Ivomec is a brand name for a veterinary formulation of ivermectin, while ivermectin is the active ingredient in Ivomec as well as in other formulations used for humans.