The term "pad" is a generic one referring to any portable personal computing device. Most often these are devices with a touch interface that has a screen that takes up a large percentage of it's surface area. Capabilities vary by use case, but an individual's personal pad will likely have communication capabilities, as well as personalization, while more generic pads are used as generic screens and devices for holding information.
The term originally referred specifically to a brand of "tablet" style computing device in the early 21st century. The "iPad" was one of the most popular computing devices ever created, leading English speakers to genericize the term to simply "pad". The popularity and convenience of these devices has kept them relevant even in the modern day, despite the huge gulf in capabilities of modern pads and those ancient devices.
The more integrated the Internet became to modern life, the less and less it made sense for there to be a distinction between a "phone" and a "tablet". Screen size and portability were the primary differences, and still well into the 21st century the distinction existed.
That changed once the first off-world colony was established. The cellular satellites that had facilitated "phone" technology didn't exist, and there seemed little reason to deploy them when a more generic solutions, a planetary 'internet' seemed a more natural solution. So, "phones" ceased to be anything more than smaller "tablets", and quickly they became just that. Eventually language caught up, and the word "phone" fell out of usage.
Generic pads are very simplistic and are generally geared to be used by anyone. They will have little to no security on them, often lack any ability for personalization, and are more like a remote terminal into the computer system they're tied to, than a "computer" in their own right.
While paper is still used in some parts of the galaxy, pads have become so ubiquitous, the more generic, simplistic ones have become a replacement for what would otherwise be printed paper. These devices are often so cheap they are essentially free, and no one thinks twice about one getting lost, damaged, or stolen.
A personal pad, on the other hand, is socially viewed as a private device. It will have security, at least biometrics, facial recognition, and often a more advanced "smart unlock" system designed to heuristically guess if the person attempting access is the owner of the device. They are encrypted by default, though depending on how expensive of a model the device is, some of the cheaper ones have known bypasses for the encryption.
It should be known that with the exception of military devices, Terran pads are required by law to have bypasses for use by the Terran government to access all information contained on them. While importing devices that don't contain this backdoor is a very lucrative business, owning one without a permit is considered illegal.
One of the things often taken for granted is that pads are almost always compatible with surrounding systems. It's possible with a gesture to simply display information from a pad on a nearby display, or to send files between devices.
This functionality doesn't always work seemlessly, however. Military systems will require the device to be authorized, meaning a civilian can't simply interact with the systems. Even then, without the right clearances, permission might be denied to copy information, or use a display.
Additionally, some civilians choose to keep their devices secured, only allowing authorized pads to access them. While this isn't common, there are those in particular fields that mean they're sensitive to the security concerns of having their personal networks kept wide open.