Here is an attempt to explain.
This is not unlike how your PC writes files to the flash memory when you transfer files from your hard drive to say a thumb drive, a form of flash memory.
One way to describe how the flash memory works...
I use a Panasonics HDC-SD9 and it uses a secure digital memory chip, another form of flash memory.
When you set the camera to record, it is "writing" its image data to the flash memory as it goes. When you stop recording, it sets all the end-of-file markings on that file and closes the file. Â
The file is typically given a alpha-numeric name that will increment with each subsequent file. So your first recording on a new memory card  could be something like HDC001.mts. The next one will be HDC002.mts  and so on.
You can then take the memory card to your PC and transfer any/all of those files to your working or storage directory folder.
The rub comes with the speed of the flash memory. I have to use a Class 6 SDHC chip or I run the risk of losing data because the camera is trying to "write" faster than the chip can record it. So it is important to find memory cards that are as fast or faster than the data transfer rate (writing speed) of the camera you are using. The owner's manual will tell you which memory cards those should be.
Life is not a guided tour nor a destination.
It is a journey. Take the time to enjoy your family, friends and surroundings.
Build memories. Share experiences. Travel at sight speed not light speed. (C)