In general terminology there is no such particular difference between Radio Button and a Select List(Dropdown) but, since the screen space used is proportional to the number of options, if the number of options are between two to seven a web designer goes for using radio buttons, for eight or more options, he/she uses a drop-down list or single-selection list.
Another thing to keep in mind while designing is: If the default option is recommended for most users in most situations, radio buttons might draw more attention to the options than necessary. Consider using a drop-down list if you don't want to draw attention to the options, or you don't want to encourage users to make changes. A drop-down list focuses on the current selection, whereas radio buttons emphasize all options equally.
The same follows for the checkbox and multiselect dropdown.