How to edit every node of a content type on cron run? On cron runs, I want to edit every node of a certain content type. I think this is implemented through hook_cron() but I can't find any documentation about this hook. How can I edit every node of a certain content type on cron runs?

I need this because I update content on nodes based on another website. I want the content to be updated regularly. I need something similar to feeds but the data source is just HTML not xml.
You would need to implement various hooks. 

hook_cron: It will get called on every cron run. Here you could load every node in the system using node_load(), edit whatever properties of the node you need to edit, and then save each edited node using node_save().

However, note that both loading and saving a node is an expensive process, and if you have a site with many nodes, your site would become very slow, unstable, or even unusable on every cron run (which can sometimes happen as often as every 15-30 minutes). In other words, I would be very cautious with whatever you are trying to accomplish.