I use categories if there is a need to list site content that belongs to a particular predefined... well, category.

Tags are not as reliable when searching content because they are not predefined. User A may use the tag "vehicle" and user B the tag "vehicles". These entities do not then show up in the same tag search listing. However if both users select the category "vehicles", the category search listing displays both of those entities.

I feel that Catagories are more for site-wide use - only Admins can do this right ?
So by their very nature they feel more "official".
They are controlled and managed.

I agree with Juho that tags can be more awkwards to find (easier to miss!?) with car / cars / Car / Cars / etc.
The tags however do feed into tag clouds - which is a visual interesting and engaging element.
Although it's not quite the same as trending (e.g. Twitter) and a tag cloud can feature tags which were once popular - maybe a long time ago.

I think both have their place, and tags are really useful for user generated content.

i'm using categories as a generalization for a post now

if you could categorise tags or tag categories, the disparity would harmonise to some extent. ;)