Omnidirectional & Directional Antennas. What's the Difference?

Omnidirectional antennas receive signals equally from all directions. Directional antennas pull in signals better from one direction. In this direction, they can detect a weaker or more distant signal than an equivalent omnidirectional antenna. The trade-off is that they do this by decreasing their ability to pull in signals from other directions. 

Gain: The difference in signal strength achieved by an antenna is known as gain, which is measured in decibels (dB). Directional antennas adjust the gain pattern to better receive signals from a specific direction. A well-designed omnidirectional antenna can also provide gain on specific frequencies the antenna is tuned to.