This tutorial shows you how to exclude current node from view. For an example, if you need a block that shows related content by taxonomy then you want to exclude current node from the list.
- Add "Content: Nid" field to "contextual filters".
- Put "Content ID from URL" as default value.
- From the drop down menu select "content id from url".
- Under "more" there is "exclude". It makes the numbers entered for the filter to be excluded rather than limiting the view.