Many players want to share their adventures with other players. So they create movies. As it is easy to upload movies on www.youtube.com many movies can be found there. You can find PK movies on here, War movies but also players doing Quests. Some people watch another person doing a quest, to make sure they won't die during the quest or get lost while looking for the right direction.
To record your adventures you can use many applications. Hypercam, Fraps, TibiCam are just a few of them. Some players even use an ordinary camera to cam their actions and put those online.
Creating a movie
When having different video fragments, one can make a movie. Windows movie maker is a free tool which is (relatively) easy to use. Read the Windows Movie Maker Manual to help you further.
There are people who SAY they uploaded a movie and they want you to watch it, so they give you a link to a site from which you can download the movie. Sometimes this document contains a keylogger and causes you to get hacked. DO NOT TRUST SITES FROM UNKNOWN PLAYERS!
Silent Defender is a well known Tibia movie maker on youtube. He has quite a few tibia movies uploaded including some of the hardest quests and showing some of the rarest items in tibia.