Does this help ?
http://www.ashtam.co.uk/what_is_an_ashtam.aspx
Official source is ICAO Annex 15 + Amendment 34, both are referenced on the page above, the bit that details ASHTAM format is Appendix 3.
Its a special form of Notam with fields that have specific meanings, the things we have seen recently in the UK at least are just normal notams that mention volcanic ash, I don't think any ASHTAMS were raised (in the UK anyway) during the recent incident, but I may have just missed them.