When the playback speed of a video on the Internet exceeds the speed at which it can be downloaded, "buffering" is employed, making the playback smooth, but delayed. Sometimes buffering can take a long time, due to a combination of several factors.
Before you can watch a streaming video from the Internet, your computer needs to download a portion of it so your video playback software has enough information to show you something. If your connection is slow, it will take longer to download the information necessary to start playback.
Sometimes it may seem as if your computer is taking a long time to buffer a video stream, when the delay is actually stemming from a bottleneck within the computer itself. Watching a streaming video requires the data to be downloaded to memory, then transferred to a temporary file on the hard disk, then sent back to memory for processing and finally to your video card for display. A failure in any one of the thousands of involved circuits will make the process seem very slow.
Some streaming video sites will allow your computer to download a stream without checking to make sure you can actually watch it. In this case, your computer might not have the proper software (or hardware) to decode and display the video stream, but it will still download the data, essentially buffering to nowhere.