Precinct Committeepersons are the backbone of the Democratic Party. They represent the Party in their neighborhood, and work with the local party leadership to help elect Democrats to public office. Committeepersons are elected to a two-year term at the Spring primary of even-numbered years. Elected Committeepersons are eligible to cast a weighted vote for party leadership at the biennial party convention held one month after the primary. You can find out more here.
The county party Chair may appoint Democrats to fill vacancies in precincts where no one was elected in the Spring primary. Appointed Precinct Committeepersons serve at the pleasure of the party Chair, but do enjoy the right to cast weighted votes in any matters before the party or their respective district.
A complete official list is maintained here.