First - backup the complete database.
Export it, save it to your computer. In fact make and keep two complete copies. Better safe than sorry.
This is really important to do.
Once you have done this you can - using phpmyadmin - select a table and from the menu at the bottom DROP it. That will remove it completely.
There is NO UNDO in phpmyadmin.
The core tables are listed here:
http://codex.wordpress.org/Database_Description#Table_Overview
As for fields - that's going to be a lot tricker and to be honest I'd leave them. They will take up hardly any space at all.
Please remember that if you see an error the only way to fix that will be to reimport that whole database that you have a backup of.
All the above said - those extra tables don't matter. They are no load to the site, the space they take up is small.