Time Machine Question

[olnacl]

After making an initial full copy of your system's drive--file by file, not sector by sector--Time Machine scans your system hourly and copies the files changed since the last scan to the external drive. The copy is non-destructive: A file is not overwritten if the archive already has a copy of it. In effect, the old file is renamed before the new copy is written. A catalog tracks the location of every file in the archive, and the time at which file was appended to the archive.

See this for more information. Hope it helps.