Linx Compatible UVC Web CamerasThanks to the standardization of the UVC USB Video Class Device specification, a wide range of Unix / Linux compatible webcams have become largely plug and play on various Linux distributions.
Manual-Focus UVC Webcams For LinuxTypically, external USB web cameras have an optimal focal range typically from a few centimeters minimum to infinity. Webcams that allow you to grab the lens assembly and turn it to fine-tune the focus might be optimal for Linux controlled robotics applications, industrial or manufacturing process monitoring, stop-motion animation, or other situations. Many document cameras that are UVC compliant like those from iPevo have manual focus to assure text is readable.
Fixed-Focus UVC Webcams For LinuxIn general, many Linux users are more than happy with a fixed-focus camera. There's nothing to futz with or think about when the camera is operating within it's optimal range. For extreme close-up situaltions, this might not be optimal. In that case a manual focus webcam with a short focal range may be preferable.
Auto-Focus UVC Webcams For LinixMany if not most mid-range to high-end Linux compatible UVC webcams have built-in autofocus which is enabled as default. It might take software to be able to intentionally disable auto-focus mode. (For example Logitech's webcam app for Windows and Mac OSX allow you to turn of AF mode.) In some environments such as those mentioned above in the manual focus section, this may be preferable.
In any environment that involves a lot of motion with either the foreground subject - or in the background - you'll often find that a UVC auto-focus webcam can endlessly struggle to adapt to ongoing changes in the frame of view. Sometimes, autofocus works against you. In face to face video chats, people who are highly animated and use a lot of hand guestures close to the camera can cause noticeable, recurring shifts in focus as the camera tries to adapt.
Linux Webcam Settings Software AppsIf you're running Linux and want to adjust and tweak your UVC webcam's image, there are a few apps available. Most notable is an open-source project QtCAM which seems to be quite popular and supports alot of image adjustment and other nice features.
Ubuntu has a good webcam resource page to check out.