In the broadest sense, antennas come in two varieties: omnidirectional and directional, sometimes referred to as unidirectional. Omnidirectional antennas, more commonly known as omni antennas, disperse radio waves in a 360-degree pattern, while directional antennas focus their output toward one direction, even though some of the signal will escape in unintended directions. Directional antennas come in four main types; sector, yagi, dish (which is also known as parabolic) and grid.
Omnidirectional antennas are perhaps the most commonly known, and can be found on automobiles, cordless phones, portable radios and many older-style cell phones. These antennas tend to have reasonably low gain due to the fact that the beam pattern, or the direction in which they can transmit and receive radio waves, is not focused. In applications where higher gain is not the primary criterion and it isn't necessary to constantly position the antenna so that it is aligned with the direction of the signal, an omni is perfect for its purpose.
Sector antennas focus most of their beam pattern into an elongated shape, allowing them to pick up gain at the expense of being able to broadcast in all directions. Typically, sector antennas are rated for beam patterns ranging from 30 degrees to 120 degrees, allowing them to make up a specific slice of a full circle. It should be pointed out that there are several names for sector antennas; in many cases, these names are more a form of branding than actually differentiating these antennas. Sector antennas tend to be flat, usually taking on a squarish or rectangular shape. Most sector antennas manufactured today are made using a printed circuit board, or PCB, with the antenna itself made up of the PCB's trace. These PCB antennas are usually enclosed in what is referred to as a radome, which is a weatherproof enclosure meant to protect the antenna from the elements.
The most common use of yagi antennas is the old-style television antennas which used to be commonly mounted on the roofs of people's homes. Yagis are a directional antenna type, and need to be pointed at the broadcast source or toward whatever antenna they are connecting to if they are being used in a two-way link.
Perhaps the most widely recognized deployment of dish antennas is by the aptly named Dish Networks. Dish antennas have a reflector solid mounted behind the actual antenna element to help in focusing the RF back towards the antenna. The older six-foot satellite dishes are another example of a dish antenna.
Grid antennas are quite similar to dish antennas, but instead of using a solid reflector behind the antenna element, a "grid" somewhat resembling a curved barbecue grill is employed. Grid and dish antennas can be purchased in extremely large shapes and are used in many applications where very tightly focused antenna patterns are required. It is important to understand that in many cases regarding the use of higher gain, very focused antennas are recommended to minimize any RF output that might self-interfere, as can happen when several radio devices share a given frequency.