There is no such thing as a “tourist” Schengen visa. Indeed, you won't find any information about “categories” of short stay visas in the regulation because there are only three types of Schengen visas: short stay visas, airport transit visas (which do not allow the holder to get out of the airport) and limited territorial validity visa (for special cases including some refugees or diplomats; you won't get one without knowing). All regular short stay Schengen visas cover a range of different purposes, including attending conferences.
On the other hand, lying on the application (i.e. pretending you are going to do some tourism when you want to attend a conference) is obviously not OK. But if you hold a multiple-entry visa and already used it for the trip you mentioned in the application, then it's completely fine to use it for other purposes.
Be aware that border guards can check that you fulfil the conditions of your visa at any time and therefore inquire about your income, purpose, etc. when crossing the border, even though you had to explain all this to get the visa in the first place. So it's good to have documents to establish the purpose of your trip in case someone asks. See Attending an academic conference is what type of travel? to know what those documents are in your case.
See also Can I use my Schengen visa for a completely different purpose and entry point? Use of unused Schengen visa to travel to Switzerland or Should my first trip be to the country which issued my Schengen Visa? for detailed explanations of what you can and cannot do with a Schengen visa in a range of related situations.