What are the different committees for in a conference?
The organizing committee (OC) represents all organizational departments of the conference, such as local arrangements, publications, publicity, satellite events, etc. Typically, for each of these departments, the OC includes one or two chairpersons. The OC is usually coordinated by a General Chair, who is the main responsible "manager" of the conference. Members of the OC usually change between different editions of the conference.

The program committee (PC) selects which papers to accept and to reject. The main task of PC members is to review papers and, afterwards, to take part in discussions of the reviewed papers. The PC is usually coordinated by one or two Program Chairs, who are actually part of the OC. Usually, the PC is more stable across different editions of the same conference than the OC; PC members often stay for 2-5 editions. 

The steering committee (SC) steers the direction of the conference, for example, by selecting the location and General Chair of the next edition, and by deciding the direction of future editions (e.g., which topics to include in the Call for Papers). Compared to the OC and the PC, the SC is particularly stable across different editions of the conference. It is often comprised of former General and Program Chairs of the conference.

These considerations assume a typical medium-sized to large conference. There can be variations, especially in small and very large conferences: Small conferences may have a scaled-down OC and no SC. Very large conferences may have several main tracks with distinct PCs, or an additional committee on top of the PC (such as a "Shadow PC" or a "Program Board").