10base5 (also known as thicknet) is the original "full spec" variant of Ethernet cable, using a stiff, large diameter coaxial cable with an impedance of 50 ohms and with multiple shielding. The outer sheath is usually yellow so it is often just called "yellow cable". The "10" means 10 Mbps, "base" means baseband as opposed to radio frequency and "5" means a maximum single cable length of 500 metres.
10base5 cable is designed to allow transceivers to be added while existing connections are live. This is achieved using a vampire tap - a device which (with sufficient practise) clamps onto the cable, forcing a spike through the outer shielding to contact the inner conductor while other spikes bite into the outer conductor. This is often built into the transceiver and a more flexible multi-wire cable carries the connection between the transceiver and the node.