A herdshare is an legal contract / agreement between a farmer and a consumer. The consumer buys into a herd for a fee. The consumer is now a non-controlling owner of a small percent of the animal or herd. The consumer then pays the farmer a fee for maintenance, care and time spent by the farmer weekly or monthly, and the consumer is then entitled to a share of the milk produced by the animal or herd she/he now owns a small percentage of.
The agreements are formal, and they are legal documents that should be constructed with careful consideration to your state laws.
Typically, depending on state laws, the consumer picks up their share at a certain date, time on the farm or at a meeting spot.
Herd Shares are not milk sales. They do provide the farmer a way to have a reasonable farm venture with dairy animals when sales or becoming a commercial dairy are impossible or undesirable.