An Apostille is a stage of attestation. In some cases it can be the only stage necessary, provided the document being processed was issued in, and is being presented in, a Hague Convention country. An Apostille contains the same information in each Hague country, but is issued by a different government body. For example, in the UK Apostilles are issued by the Foreign, Commonwealth & Development Office (FCDO), and in the Republic of Ireland they're issued by the Department of Foreign Affairs (DFA).
If you're presenting the document outside of a Hague Convention country, it will need further attestation. The UAE is an example of a country where this is required, therefore if a UK document is destined for the UAE, it will need a further stamp applied after the Apostille by the UAE Embassy in London.