i was watching a video by blender guru and i saw that he was trying fairly hard on keeping all the faces on the mesh he was working on to 4 vertices. think he mentioned that its so that each edge will have a direction or something similar. other than that will it really affect the image quality or show any artefacts in the final render?
I've always found that triangles are thought to be bad in just about every single case, while this simply isn't true. There are some pros and cons to using triangles. They're like any tool, they have their uses, and also places where they should and should not be used.
Triangles cause a few issues that make modellers and animators dislike them.
They do not subdivide well, and usually cause pinching or other ugly results.
They interrupt edge flow, which is what Andrew (Blender Guru) is referring to. A loop of faces has to be made entirely of quads, as a triangle will terminate the loop (note that this can be a good thing in some cases).
Overall, triangles just make the mesh a bit more messy and less easy to work with. However, they aren't always bad. There are some advantages to triangles as well.
Triangles allow the artist to keep vertex count low by only adding the necessary geometry.
They can terminate edge/face loops and make changes in mesh density easier.
They can let the artist view the exact mesh shape that will be rendered (GPUs can only deal with triangles, so all objects are triangulated prior to rendering). Usually this issue is only run unto when your object has large faces, such as in low-poly art. An example of this can be found here.
They can assist in deformation as no matter how you reshape a triangle it will always remain convex and flat, whereas quads or n-gons can be deformed in such a way that a face can exist outside of the actual edge boundaries.
If you've ever worked with professional game models you will also notice that they are all triangles. This is to allow the game engine to skip converting the model to triangles upon rendering, as well as allowing the artist to have as much control as possible over how the faces are triangulated. Triangles also help the artist keep the polycount within appropriate limits.
In summary, if you know what you're doing, triangles can be extremely useful. However, when working with models it is usually easier to keep them all as quads unless you are at the final stages and no subdivision will be used.
It makes a number of things easier and some modifiers only work on quads. One such modifier is subsurf. Also when you need to make a loop cut, it only works on quads. When you hit a triangle or ngon the cut you are making will break there.