Legalisation confirms that a document was issued by someone with the authority to issue it, and that the signatures it bears are genuine. Some documents need to be signed by several different authorities in order to be legalised. This is called the legalisation chain.
Some countries have entered into agreements streamlining the legalisation chain. This means that certain documents from one of the contracting states may be used in another contracting state with only a single legalisation or even none at all (mainly within Europe).
An authorised official issues the apostille once he/she is satisfied that the document and its signature are genuine. In most countries, the apostille is issued by the Ministry of Justice or the Ministry of Foreign Affairs.
The apostille comprises one or more rubber stamps and two or more signatures.