Essentially both are just conditioner, the difference is how they are used:
There are 'cleansing conditioners' which are marketed to curlies and usually contain ingredients to make them effective cleansers, but they still condition (ie moisturize, provide slip, soften hair).
'Regular' conditioners generally include some cleansing ingredients too but any condish can be used as a cowash because its really the friction of massaging it on the scalp/thru hair that cleans. The same is true for any cleanser (soap, shampoo, face wash).
So in general; a cowash is whatever conditioner one uses to clean the scalp and hair, it is usually applied to the scalp, massaged well, rinsed out with more scalp massage. A rinse out is a conditioner that is applied, left on without massaging scalp, then rinsed out. A leave in condish is any condish put on and left on after which the hair is dried, styled, etc.
Pretty much any type/brand conditioner can be used for any of these uses.