Sometimes a picture is worth a thousand words, Jin. Here are some pictures of "grinding machines", which can also be called "grinders". Here are some pictures of "grinding wheels". Both grinding machines and grinding wheels are designed to remove excess material from something with abrasion.
Remember that all these words are derived from the word "grind". As long as you understand that verb, you should have no problem understanding what "a grinder", "a grinding machine", or "a grinding wheel" is.
Linked to first page of Google Images for "Grinding Machines" and "Grinding Wheels".