What are they?

From 1999 onwards most new Public Acts are published with Explanatory Notes.  These are written to help those without legal knowledge or expertise in that area of law to understand the purpose of the act and to give it context.  They also have a table at the end which lists the Hansard references for that piece of legislation.  More information can be found on the HMSO website at http://www.hmso.gov.uk/legislation/uk-expa.htm

Where can I find them?

Explanatory Notes are published by The Stationery Office (TSO).  They can be found electronically on the Legislation.gov website by searching for the act  needed and clicking on the act title.  The notes should appear as a separate tab at the top.  You can also find them on many of the subscription databases.  On Westlaw they appear at the end of the act (if you download the PDF) or at the bottom of the table of contents.  In Lexis Library there is a source called Explanatory Notes for Primary Legislation (under Commentary, not Legislation) which you can search and browse.  On Lawtel if you search for the act and click through the Explanatory Notes are a link from that page.

Do all post - 1999 acts have Explanatory Notes?

Not all acts after 1999 will have explanatory notes, the main exception is legislation that was introduced to Parliament by a Private Members Bill.  For these bills the Explanatory Notes are optional.

What about before 1999?

Before 1999 there were no Explanatory Notes published with acts.  There are often Explanatory Memoranda published with the bill which explain the bill contents as it enters each House.  These again are published by the TSO.