Whenever communications to/from a space vehicle are described or discussed, there is inevitably a reference to the vehicle's "high gain" antenna. What is it about such an antenna (specific technical features) that makes it "high gain", and what alternative equipment is fitted to a spacecraft that would require use of the qualifier "high gain"? Is there usually some sort of low or not-so-high gain antenna fitted?
A higher gain antenna is more directional than a low-gain one. In any given situation use of a high-gain antenna permits higher data rates but requires more accurate pointing of the antenna toward the other station. In turn this requires that the spacecraft attitude determination and control be operating correctly. If the HGA is fixed to the body, this also constrains the spacecraft attitude while using the HGA.
In most cases a spacecraft will have both high and low gain antennas - high will normally be used for high-speed communications, and low in emergencies or during commissioning before attitude control is established.
"HGA" and "LGA" are relative terms, and the HGA could be anything from Cassini's 4 meter diameter parabolic reflector to the quad array of ~4cm helical X-band antennas on Planet Labs' earth observation cubesats. The LGA is usually somewhat omnidirectional, though because of the hairy ball theorem no antenna can be perfectly omnidirectional. A monopole or dipole construction is typical (though the radiation pattern will differ from the classic mono/dipole patterns due to the spacecraft structure) or it could be a low-gain patch antenna which might still require some degree of spacecraft pointing. Multiple low-gain antennas can be used, either multiplexed or connected to separate transceivers, to get better diversity of coverage.
A high gain antenna might be deployable in order to achieve a large aperture while still fitting inside the rocket fairing, though after the Galileo failure designers may be nervous of that. It could also be steerable either mechanically or electronically. The LGA is usually fixed, both for reliability and because it doesn't need to be physically or electrically large, and doesn't need to be pointed.