Via types

IPC-4761 (Design Guide for Protection of Printed Board Via Structures) defines several types of methods for protecting PCB vias. These are:

Type I: Tented
Dry film solder mask is stretched over the hole. No other materials are added.
Type II: Tented and covered
An additional dry film or liquid solder mask layer covers the tenting of a Type I tented via.
Type III: Plugged
A non-conductive material penetrates partially into the hole, but does not go all the way through.
Type IV: Plugged and covered
An additional dry film or liquid solder mask layer covers the plugging of a Type III plugged via.
Type V: Filled
A conductive or non-conductive material penetrates completely into the hole, filling the hole all the way through.
Type VI: Filled and covered
An additional dry film or liquid solder mask layer covers the filled material of a Type V filled via.
Type VII: Filled and capped
A metalized coating covers the filled material of a Type V filled via.

In general, production costs increase as you go down the list, with type III being more expensive than type I, etc.

Additional info: