Basically, when you wash your hair you are stripping away your natural oils (sebum). Not good. Well, if you didn't you would have dirty, greasy hair. Unfortunately, our natural oils like to attract natural dirt.
After shampooing, a good conditioner can restore the oil back to your hair. Humectants attract and hold moisture in your hair. Many conditioners also contain hydrolized keratin which is a protein that will penetrate your hair to strengthen it. Acidifiers in conditioners restore balance and give your hair elasticity and shine. They keep hair from being "weighed down".
Conditioners can be divided into six categories. Here is the quick lowdown.
Reconstructors - contain protein
Acidifiers - contain acid, about 2.5 to 3.5 ph
Detanglers - usually contain acids
Thermal Protectors - contain polymers that protect hair from blow drying, hot curlers, irons, etc.
Glossers - contain silicone-derived oils to make hair shine. Purely cosmetic.
Many men's hair product lines carry both shampoos and conditioners. Always buy the smallest quantity when first trying out a product. See if sample products are available. Also, use the product a few times before making a judgment, especially with shampoos and conditioners. Test it out for 2-3 weeks, but if you get nasty side effects after the first use stop using it. This usually never happens, so make sure you buy a shampoo/conditioner that is made for your type of hair.