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(1) Through hole.
(2) Blind via.
(3) Buried via.
The gray and green layers are nonconducting, while the thin orange layers and red vias are conductive.
A via (Latin, 'path' or 'way') is an electrical connection between two or more metal layers of a printed circuit boards (PCB) or integrated circuit. Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with metal (often copper) that forms an electrical connection through the insulating layers.
Vias are an important concern in PCB manufacturing.[1] As vertical structures crossing multiple layers, they are specified differently from most of the design, which increases the chance for errors. They place the strictest demands on registration (how closely aligned different layers are). They are manufactured with different tooling from other features -- tooling that typically has looser tolerances. If either the hole or any layer is slightly out of place, the wrong electrical connections may be made; this may not be visible from the surface. After the hole is drilled, it must also be lined with conductive material, as opposed to simply leaving conductive material in place on copper layers. Even an initially good board may develop problems later because the via reacts to heat differently from the substrate around it. Vias also represent a discontinuity in the electrical impedance, which can cause problems for signal integrity.