Parasitism is when one organism benefits at the cost of it's host. Like fleas on a dog or intestinal parasites to livestock. Fleas and intestinal worms get food and a nice place to livr, while the dogs and the livestock are being leeched off of and malnourished.
Commensalism is when one organism benefits, while the host is left unharmed- but also doesn't really get anything in return, unlike mutualism. Like a shark and remoras (those fish with suction-cup heads & hitchhike). Sharks aren't hurt, but the remoras get free transportation and access to food scraps.
Hope this helps!
