Vias are one of the major parts of any printed circuit board that has two or more layers. Its main job is to electrically connect at least two paths or area (plane layer, copper area etc.) in different layers by a hole through the layers. Vias are also used for thermal conductivity. The hole is usually made conductive by electroplating. There are different kind of vias like; through hole vias, buried vias and blind vias.
Through hole vias: This kind of vias have a path through all the layers.
Buried vias: This kind of vias have no path to the external layer(top and bottom).
Blind vias: Has a path to only one external layer.
When routing a wire, if high number of vias are used the parasitic capacitance and inductance and impedance of the path may cause problems in high speed pcb designs. So try to avoid using vias on high speed signal paths especially in clock signals. Smaller vias have smaller parasitic capacitance so use smaller via pads in critical tracks. When routing differential signal traces try to keep number of vias on both line the same.
Minimum drill size of vias is an important parameter for the pcb manufacturers.
Cost of PCBs that has blind/buried via technologies is higher comparing PCBs which only use the normal through hole Vias.