Tapeless HD camcorders record a specific video file to a card or a drive connected to the the camera. This file is encoded with a specific video compression/codec.
In Final Cut Pro “Log and transfer” will transcode the format (AVCHD most likely) you shot to the card or drive to Apple ProRes 422. The quicktime mov files will be stored wherever you set the scratch disk in FCP menu, system settings.
The Apple ProRes 422 codec provides the following:
Target data rate of approximately 145 Mbps (1920 x 1080 at 60i)
You can select many different options in the import preferences. The preset is Apple ProRes 422. There is a higher preset (HQ) with a higher data rate, but you would use higher settings only if you have a camera that records with higher color sampling and more resolution. I never change the preset.
Click the sprocket in the log and transfer window to see the different preferences.
Once the mov is transcoded it will be in the browser, then drag it to the timeline.
FCP installs with a preference that will make any new sequence NTSC DV (standard definition video). There is another preference that will pop up when you drag an HD clip to the timeline asking you to change sequence settings to match clip settings. If you click yes, you will be editing in the native format of the clip. If you want the clip to be forced into a letterbox 4:3 sequence (scaled 50%) click no. It all depends on what format you want to export.
For more about AVCHD:
For more about color sampling:
FCP Log and Transfer Tutorial: