A drive-by-download can refer to several things.
- A download that happens with the user's consent, but they didn't really understand that the consequences.
- A download that happens without the user's consent or knowledge
The first method is not very "drive-by" in my opinion, but it is still useful to recognize that this is still possible.
Tricking someone into downloading a file can be as simple as them thinking they are clicking a regular link and then downloading an EXE, PDF, etc. You don't need JS or a plugin to do this, but it helps. A very simple example would be a "popup" (just a piece of html styled with CSS to look like a popup) that says "you have a problem with your computer, click ok to download a fix." Clicking this would initiate the download of an EXE, that the user may or may not run.