The short answer is ‘not a lot’.
The main thing is that they are ‘digital’ so they can be accessed from any device, anytime and from anywhere. Well, at least they should be all these things, but not all attendance register apps or register software is created equal (more on that in a minute).
There are lots of variations, but Apps are generally an ‘application’ that sits on your mobile device, whilst software is something that runs from your laptop or computer.
The exceptions are pieces of software that are hosted in the cloud and these are known as Software as a Service (SaaS) providers. SaaS platforms are becoming far more commonplace as they’re so much better than traditional software. Most, if not all, pieces of modern-day attendance register software are SaaS based.
Older or larger types of attendance register software (sometimes seen in schools for example) are based on computers physically located within an organisation and usually have lots of computers speaking to each other (networked). They often use a central server and are usually very complex to set up and use, and very expensive to buy and maintain.