Second Step: Explaining Bleeds
Explaining bleeds is very simple: Bleed is a printing term that is used to describe a document which has images or elements that touch the edge of the page, extending beyond the trim edge and leaving no white margin.
When a document has bleed, it must be printed on a larger sheet of paper and then trimmed down.
The minimum amount of bleed should be around 0.125" (3mm) outside your document final size, ideally 0.25" (6mm). Each printer has his own requirement for this. The only time you don't need to use bleed is when there is absolutely nothing printed on all sides (eg. a design with a white border.) In this case, you can simply provide your final print‐ready at its final size, without any extra bleed.
Some small ads in magazine or newspapers don't require it but it's better to provide a file with some bleed if you're not certain.
Another situation where you might not need to add bleed on all sides is when your file is provided as a “printer spread”; for example, a book cover design with the spine, front and back cover on the same layout OR a brochure with a fold OR a greeting card.
The bleed will only be necessary for the printer on the outside edges. It's still a good idea to create your own designs (front and back cover) with bleed on all sides; it will be easier for you to merge them together later when you'll prepare the final print‐ready file.