Custom written software is developed and produced to meet your requirements. Whether it be an antivirus, stocktaking software, or software for recording weights. This software is almost boundless. It can stretch as far as your imagination, and your developer's skills allow.
The more complex the software, the more talented your designer must be, resulting in higher costs. But, the simple answer is that custom software is coded, designed, and produced according to your specifications.
A hybrid of software exists, where companies customize off the shelf platforms.