Many computer systems have built in storage, meaning that the device used to let you store files is built into the hardware. It cannot be moved without moving the entire system.
Portable storage is not built into a system and can be carried by a user and used on many different systems.
Most devices also allow you to attach external hardware devices to them, such as a memory stick or external hard drive. This allows for an increased storage capacity. These devices can be attached to other computer systems, which means that your files are portable.
Portable devices are small and easy to carry around. For example, USB flash drives and flash cards (found in mobile phones and cameras) are extremely small, with high memory storage capacity and have no moving parts.
Magnetic tape is now largely obsolete for personal use but is still used for backing up server data in industry.
Portable devices are compatible with all computer systems because they mostly use a USB (Universal Serial Bus) interface.