The short answer:
District 71 is a geographical subset of the Northeast Texas Area 65 of the AA General Service Conference. Its boundaries are defined in the map found here.
The long answer:
AA is a fellowship having one primary purpose–to carry its message to the alcoholic who still suffers. Anything which supports that effort is a service. Services are rendered individually, by collections of individuals, and by the general service structure. That structure is composed of AA groups, districts, areas and the General Service Conference (GSO). AA general service resources can be found on the GSO website here, along with the AA Service Manual, which describes in a generic way the group, district and area activities and their function within AA General Services.
District 71 is both a service concept and an implementation of the AA General Service Structure. As a concept, it describes the activities within the geographical area described above. Any individual engaged in AA service within this area is welcomed to participate in District 71 activities. Anyone is welcome to attend District 71 meetings. Each AA group within the district boundaries is served by District 71.
As implemented, District 71 follows the outline for a district as described in the AA Service Manual, Chapter 3. Each group GSR is a member of District 71. The district sponsors several well-defined committees with a focus in one service area, like Treatment Facilities, Public Information, Correctional Facilities, etc. The chair of each of those committees is a member of District 71. The district requires the efforts of a Secretary and Treasurer who are also members of District 71. The district is lead by the District Committee Member (D.C.M.) who also serves as a formal link to Area 65.