Drive-by attackA drive-by attack typically involves downloading a malicious program without consent, mostly just by visiting an infected website. A hacker finds a vulnerability in a website and places a piece of software on it. When a visitor comes by, the software scans the users' browser for vulnerabilities that it can use to install the software on the persons' computer. Sometimes a drive-by attack involves clicking on a link or some sort of a button (like an "x" in the corner of an advertisement) that does something else than it says it does, hence: the visitor is unknowingly tricked into installing malicious software on his own device.
Using safe and updated browsers in combination with safe-browsing software are good ways to protect yourself against these types of sneaky attacks.