vLANs, or Virtual LANs, are a way to logically, rather than physically, segregate your network traffic. This can be done for multiple reasons, but primarily is done for security. Traffic from one vLAN cannot cross over to another without a static route existing on the router. Each individual port on a switch that supports vLANs can be configured as a member of one or more vLAN. The port itself can be configured as either an access port, which allows ONLY membership on a single vlan, and does not require setting up your device to support vLAN (tagging your traffic), or a trunk port, which can be a member of multiple vLANs simultaneously. vLAN traffic can either be untagged or tagged. These concepts will all be explained below.
Switch port types
Access ports are intended for individual network devices, such as user PCs, phones, etc. These ports are members of a single vLAN, and ALL traffic received from the plugged in device is automatically considered to be part of that vLAN. This has the advantage of allowing devices to participate in a vLAN that do not natively support tagging. The disadvantage is that you cannot make use of multiple devices coming off of that single port that need to be on different vLANs, such as a phone on a voice vLAN, with a PC plugged into the phone's PC passthrough port, on the data vLAN.
Trunk ports can carry multiple vLANs simultaneously. By default, they are generally configured to carry ALL vLANs, but can be restricted to specific ones in the switch vLAN port configuration section, depending on switch make and model. All trunk ports have a default vLAN membership, which is what vLAN untagged traffic is communicated on, and all other vLAN memberships must be tagged, so that the switch can know which vLAN to pass tagged packets to and from. If a device with a vLAN ID is connected to the port that does not match any of the port's membership IDs, it will not pass any packets tagged with that ID at all.
All network traffic, unless otherwise specified, is untagged by default. This simply means it has no vLAN header at all, and lets the switch transmit this traffic on whatever the default vLAN that particular port is set to (see above in access and trunk sections).
Packets with an optional vLAN header contain a VID (vLAN ID), which tells the switch what vLAN this packet belongs on. If the port is not a member of that particular vLAN, it will not pass the packet (in or out). This prevents someone from simply configuring a computer with the correct IP address information from accessing that network, if they are not plugged into a member port of the target vLAN, even if they do have the correct VID configured on their computer.