When we download video from Youtube or any other website, 90% chances are you are watching them using Flash Video Player whose format is FLV. Some of the videos uploaded in Youtube are sliced into multiple parts especially when Youtube only allowed you to upload a video of maximum 10 minute duration.
There is a simple but useful way of joining these files into a single under Ubuntu.
First you will need to install the package:
sudo apt-get install avidemux
Next, keeping the audio/video encoding unchanged, just join them (change the filenames accordingly):
mencoder -forceidx -of lavf -oac copy -ovc copy -o output.flv clip1.flv clip2.flv clip3.flv