End of January 2021, Esri launched a new product called “ArcGIS Platform”, which is a “platform-as-a-Service” (PaaS) offering. “Platform-as-a-service” means that Esri provides the infrastructure (servers, etc.) needed to run, and the developer tools required to build, applications that use ArcGIS Platform.
Not surprisingly, the services ArcGIS Platform provides revolve around “location technology”. They range from traditional location services like geocoding or routing to no-code or low-code GUI builders (like ArcGIS ExperienceBuilder) and to data (for example demographic data; you can use the “Data browser”  to explore what data is available).
What is new?
There is a whole new developer portal which serves as the “entry point” to ArcGIS Platform. Developers can sign up for free and start using ArcGIS Platform instantaneously, free of charge. The portal also provides extensive documentation. I cannot tell from experience yet, but at first glance it does look very comprehensive.
In contrast to other products, ArcGIS Platform uses a “pay-as-you-go” business model. This implies that costs apply to transactions, i.e., usage of the services, not general access to them. And quite generous volumes of the free tier apply before any costs accumulate at all. The free tier includes, among others, consumption of 2 mio. basemap tiles and execution of 20,000 routing queries per month. According to Esri, there are no plans to adjust these number in the future.
As a consequence of the “pay-as-you-go” scheme, the use of Platform services requires authentication. Mapping every request to a developer account is required to track usage per account. Esri’s Platform is following industry standards for authentication and allows the use of API keys (basically passwords). This is in addition to the existing support for OAuth2/OpenID Connect.
Platform also supports third-party mapping engines (e.g. Leaflet). And it is generally easy and quick to integrate into other products in order to provide them access to location services.
And finally, it is worth mentioning that Platform also provides support for hosting (your own) data. If you choose to do so, the data remains your data, Platform is just hosting it.
How does Platform differ from ArcGIS Online (AGOL)?
Facing this new product may create some confusion with regards to the differences between Platform and the existing ArcGIS Online (AGOL) product line in the community. Me included, initially. But bear with me: I will have a go at showing the differences between the two here. The following screenshot is from the plenary session of the Esri DevSummit.
Different Business Model
The pay-as-you-go model is different from the licensing model of ArcGIS Online. As mentioned above, Platform includes a generous free quota of service usage before you incur costs.
Platform is a subset of Online
Platform powers a lot of Online. Esri uses their own product in their other products.
It focusses on services
There are no apps on Platform. Platform provides services. If as a vendor, you sell an app that uses Esri technology, it will not be via Platform (although the app might use services from Platform).
Support for third-party products
Somewhat orthogonal to the case of selling an Esri-based app: if you are selling a non-Esri-based app that requires location services, for example for Leaflet, you can now use ArcGIS Platform to consume these services from your app.
Wrap up – tl;dr
With the launch of a product called “Platform” in January 2021, Esri moved towards paradigms known as or similar to pay-as-you-go, service-oriented architecture and micro-services. With a very low entry threshold, developers get access to a broad range of services at costs that scale with usage. If you always wanted to get a quick glance of the capabilities of Esri’s products, “Platform” just made this a lot easier and more convenient!