NICs are considered Layer 2 devices because each individual NIC throughout the world carries a unique code, called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network. You will learn more about the MAC address later. As the name implies, the NIC controls the host's access to the medium.
In some cases the type of connector on the NIC does not match the type of media that you need to connect to. A good example is your Cisco 2500 router. On the router you will see AUI (Attachment Unit Interface) connectors and you need to connect the router to a UTP Cat5 Ethernet cable. To do this a transceiver (transmitter/receiver) is used. A transceiver converts one type of signal or connector to another (e.g. to connect a 15-pin AUI interface to an RJ-45 jack, or to convert electrical signals to optical signals). It is considered a Layer 1 device, because it only looks at bits, and not at any address information or higher level protocols.
NICs have no standardized symbol. It is implied that whenever you see networking devices attached to network media, there is some sort of NIC or NIC-like device present even though it is generally not shown. Wherever you see a dot on a topology, there is either a NIC or an interface (port), which acts like at least part of a NIC.
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