Synchronization in Location Recording   

single system 
For simple video shoots, synchronization is not an issue. The video recorder or camcorder will record audio directly to the video tape and synchronization of the raw footage is assured.  It is interesting to note that before the era of ENG or electronic news gathering using video camcorders, there were single system film cameras which used film specially striped with a thin magnetic track so that the camera could record sound as well. 
double system 
For film or more involved video shoots, a separate audio recorder is used and thus the need for sync systems on location arises. 
Film (Double Systems)
In North America film cameras run at a precise 24 frames per second (fps).
Cameras used for synchronous sound recording are designed to be quiet or are blimped and run at a precise speed.
Up until recently, sync was achieved by recording the audio onto a sound recorder with an equally precise speed control, the theory being that in playback the record accuracy of both devices would insure correct lip sync.
Coordination of image and sound is achieved by means of a clapper. The recorder captures the sharp "clack" and the camera the image of the two clapper sections meeting. Aligning the picture frame which contains this image with the sound frame which registers the sound recreates the original sync. This is called a slate and a tail slate or slate at the end of the shot may also be used as a further indication of sync accuracy.
Recently, time code has been added to this process. The camera (either film or video) generates time code and/or an electronic clapper is used which displays the time code. The audio recorder uses time code to maintain speed accuracy during recording & playback. This not only insures perfect sync but also makes finding the appropriate section of sound in post production much easier. In low budget situations, where a professional audio recorder is not an option, one may attempt "wild sync" for short sync shots. Using a reliable audio recorder and a clapper, the synchronization may be serviceable.
There are currently several audio recorders used for location sound recording:
 Nagra    DAT    Nagra Digital    Walkman Pro