A hitchhiker is traveller who relies on free rides and transport from other people as their means of getting around. Hitchhiking is one of the themes of The Hitchhiker's Guide to the Galaxy, and even features in the title of the story.
Interstellar hitchhikers are cosmonauts who travel around the galaxy by hitchhiking on interstellar ships. The hitchhiker does this by flagging down passing spacecraft with an Electronic Thumb, an electronic sub-etha signalling device. Hitchhikers can determine if there is a nearby spacecraft using the Sub-Etha Sens-O-Matic, which is a small black device that alerts hitchhikers to passing spacecraft by blinking.
These hitchhikers always try to carry a towel, which was about the most massively useful thing an interstellar hitchhiker could have. They also often carry a copy of The Hitchhiker's Guide to the Galaxy which aids them in their travels by providing information on the places they might find themselves and the things they may encounter. A lot of the information in the Guide revolves around the best places in the Universe to drink, and what things to drink there, as getting drunk is a favoured past-time of many in that profession.
Many interstellar hitchhikers worked for the publishing company behind the Guide, Megadodo Publications, as they were the best people to know what information would be most or least useful to hitchhikers travelling around the Universe. Interstellar hitchhikers were known to go on research trips for the Guide - as one Ford Prefect did - and to frequent the reception area of Megadodo Publications.
Ford Prefect was the first interstellar hitchhiker introduced in The Hitchhiker's Guide to the Galaxy series. He journeyed to Earth via a teaser as his editor sent him to the planet for research purposes. He then gets a lift off the planet via a Vogon ship, with Arthur Dent in tow. Not long after this, Arthur agrees to join Ford in his hitchhiking and researching and becomes an interstellar hitchhiker himself.