I am using bootstrap.css and I found that there is another css file named bootstrap-combined.min.css, is there any difference in style? Is there any need to include both css files? Which one is more preferable?
The file bootstrap-combined.min.css is for Bootstrap v2, it is the combined css file of bootstrap.css (the main css file) and bootstrap-responsive.css (the responsive Bootstrap styles). It's exactly the same as using those two files separately but it just saves you an extra request.
The "min" part of the filename just means that it has been minified, which is to say that it has had all the unnecessary white-space/comments/etc removed.
Conclusion
bootstrap.min.css = compressed version of bootstrap.css
bootstrap-combined.min.css = bootstrap.min.css + bootstrap-responsive.min.css