Online videos usually play with the media player you have installed. If you didnt have the correct player installed it would tell you when you go to click play, bysaying you need to download Windows
Media player for example.
There are three main problems that can cause this:
Firstly, you media player that you currently have installed may be out of date. Updates are normally free at the main site for which ever media player you have.
Secondly, slow internet connections can lengthen the time it takes for the video to download -as essentially thats what the buffering is, it downloads all the information to your computer to enable you to play it.
Thirdly, that particular site may be receiving alot of traffic at the particular time you want to play items. Websites run on bandwidth, and if there are alot of users playing the media at the same time as you, the bandwidth is quickly 'eaten up', which slows everyones downloads
I find I get this problem alot at certain times on you tube. The best way to combat this is to press pause and wait for the whole of the buffering process to take place before you press play. This will give you an un-interrupted play of the media.
I usually open a second window and select the next item I want to play, pausing it and letting the buffer run. By the time I've done this I usually find the first is ready to go.
Sometimes they enable you to play items withi an install
of an active X component - again if you dont have the right one you will get a pop up asking you to install it.
The best solution is to upgrade your media players for free on the main sites, and then just press pause until the media has buffered. I often get this problem as mentioned above on sites which have a low bandwidth, even though I have a high broadband bandwidth. This is simply because their bandwidth cant cope with the demand of many people playing at once, high bandwidth costs exra money for them to get on their site, so smaller and larger sites are well known for only having just enough of it to enable people to view the media, but in the process it slows down their bufering processes.