What is a Via, Micro via, buried via and blind via in PCB design?

Via

In PCB Design a via made an electrical connection between two or more layers. Its visible from both side of PCB.

Yes, its more like a Pad which has a drill hole. But drill dia for Via is smaller as compare to Pad. And that is the only part which distinguish it form Pad.

Blind via

It's start from one of the surface to any of inner layer.

Burried Via

It's via that is between inner layers. burried via is not accessable from top or bottom layer.

Microvia

These are the vias with narrow drill hole. usually these vias are directly present in component pads.

Reference:-

1.  Reduce Your PCB Costs With Blind Vias, Buried Vias & Microvias  Retrieved on 11 Oct   2014 http://www.eetimes.com/author.asp?section_id=36&doc_id=1320862