Official Sinn Féin (aka "Sinn Féin the Workers Party") evolved from the split in Sinn Féin and the IRA that took place in 1970.
The leadership of both Sinn Féin and the IRA had developed a Marxist outlook that became unpopular with many more traditionalist nationalists/republicans; and the party/army leadership was criticized for failing to defend northern Catholic enclaves from loyalist attacks. A large segment of the party set up their own Provisional Army Council in late 1969 to address these concerns and draft a new party policy; however, at the January 1970 party assembly, the "Provisionals" failed to secure the necessary votes to change the party direction. Immediately, the Provisionals left the existing party, thereby forming the Provisional Sinn Féin and Provisional IRA. The remainder of the original organization renamed itself as Official Sinn Féin (and the paramilitary arm as Official IRA).
The Officials gradually stepped down their military campaign against the United Kingdom's armed forces presence in Northern Ireland and focused on electoral activity. Official Sinn Féin evolved into the Workers Party which has had some electoral success in the Republic of Ireland but little in the North, although they still do contest some seats. It is still more of a Marxist party than a Republican one and is avowedly non-sectarian.
The Workers Party suffered further splits in the 1970s, spawning such groups as Democratic Left which later became part of the Irish Labour Party; Irish National Liberation Army; and Irish Peoples Liberation Organization.
Despite the numerous splits, and large migration to Workers Party, Official Sein Féin is still said to exist as a separate entity.
Provisional Sinn Féin is the party that is now commonly referred to simply as Sinn Féin and they have had much greater electoral success than the Official group.
The Official party was nicknamed the "stickies" for wearing Easter lily stickers on their lapels in remembrance of the 1916 Easter Rising.