A regular dedicated server is a dedicated web server where you do all the managing of the dedicated server yourself.
You take care of the software updates, operating system updates, installing things that your website needs to run such as PHP, MYSQL, Mail Server, Security etc.
With a Managed Dedicated server the hosting company will take care of everything on the server for you so you won't have to keep the server up to date or do anything to maintain it.
If you don't know how to operate a dedicated server then you should hire someone or go for managed dedicated hosting instead.