What is an ArcGIS Developer Subscription ?

The ArcGIS Developer Subscription provides developers with access to Esri API's and SDKs, as well as developer tools and services that supplement the process of application development with Esri technology. As with numerous other Esri offerings, the developer subscription is available via a variety of plans, starting with the "Essentials" level. The Essentials developer subscription is obtained by signing up for a developer account at https://developers.arcgis.com/sign-up.  - This is a freely available Subscription, and provides the developer account holder with access to the inclusions outlined below.

Beyond the "Essentials" level, there are a further four purchasable ArcGIS Developer Subscription (ADS) plans available, each with increasing  inclusions, beyond those included with Essentials. These plans, in order of increasing  product inclusions, are : "Builder", "Professional", "Premium" and "Enterprise". Inclusions are cumulative, meaning that each plan will include all entitlements specified in the level beneath it; So, as an example, all Professional inclusions will also be found amongst the Premium inclusions.

The key difference between a free Subscription and a purchased Subscription is that a purchased subscription comes with its own set of core, off-the-shelf Esri products, such as ArcGIS Pro or ArcGIS Enterprise (eligibility for which depends on the purchased level). The inclusion of such core products allows for the setting up of distinct Esri development and testing environments, where various workflow and deployment scenarios may be configured and evaluated.

ArcGIS Developer Subscription Inclusions

The ArcGIS Developer's pricing page, "Building ArcGIS Solutions" tables the Developer Subscription inclusions, increasing from the "Builder" plan, through to the highest "Enterprise" plan. What though, of the distinction between free Essentials inclusions, and Builder inclusions? - Tabled below is an elaboration of the above "Building ArcGIS Solutions" link; It incorporates inclusions at the Essentials level, therefore providing a clear summary of distinctions between the free and purchased subscription levels.

In general, all ADS plans provide access to all Esri API's, SDKs and app builders, excepting the ArcGIS Pro and ArcGIS Enterprise SDK's which a developer would only require, should they already have licenses for ArcGIS Pro or ArcGIS Enterprise. The ArcGIS Developer's Downloads page is used to access respective API/SDK download links.
There is also an evident distinction between the Essentials ADS and the Builder ADS, where a developer seeks to use App Studio and deploy developed apps via the Apple App Store, or Google Play. - This is not possible with an Essentials plan, but is possible for purchased ADS plans, where a "Developer" AppStudio license is provided.
Another notable difference, the Essentials subscription includes Esri Community support which comes in the form of access to various online Esri forums. Beyond this, more personal one-on-one support is available via "Esri Technical Support"; This though, is only available to holders of a purchased ADS.
Further, all purchased ADS plans include access to a distinct 5 member ArcGIS Online (AGOL) Dev and Test organisation, which exists for the purposes of emulating a production AGOL environment, and testing planned workflows. This is distinct from the single member AGOL account which comes with all developer subscription plans; This single member AGOL organisation could primarily be seen as a web GIS content creation and hosting facility.

In terms of accessing license files for core products provided under an ArcGIS Developer Subscription, Esri's MyEsri user portal is used by the developer assigned to the subject ADS. Since these inclusions are only applicable to purchased ArcGIS Developer Subscriptions, using MyEsri in this way is not applicable to the Essentials level developer.

The concept of the ArcGIS Developer Subscription, where developers have access to a full suite of Esri product, for "dev and test" purposes at a heavily reduced price, has been around for many years; Some readers may even remember the "Esri Developer Network" which was a similar "predecessor" to the ADS. Esri developers will also likely know, the experience of the ADS holder is fundamentally based around tools and resources found within the ArcGIS Developer's website : https://developers.arcgis.com.
Within the last 12 months though, the release of "ArcGIS Platform" saw the availability of many new capabilities for ArcGIS developers1, and consequently the ArcGIS Developer's website was extensively overhauled to accommodate these changes. - This is the reason that https://developers.arcgis.com took on a very new appearance in late January of this year, and therefore, that ADS holders (whether holding a free or purchased subscription), had access to a whole a new world of ArcGIS developer-centric services and tooling.
1 :New capabilities that became available to ADS holders when ArcGIS Platform was released are denoted with an * in the below table.

ArcGIS Developer Subscription Inclusions, by Subscription :

Further to the above table, Esri core products (eg. ArcGIS Pro, ArcGIS Enterprise), ArcGIS user type extensions (eg. Utility Network), ArcGIS premium apps (eg. ArcGIS Collector), and extension products for ArcGIS Desktop and ArcGIS Enterprise  (eg. Spatial Analyst, GeoEvent Server) are *only* available to holders of purchased ArcGIS Developer Subscriptions, depending on the purchased level, the variation of which is listed here.

Summary

The ArcGIS Developer Subscription is accessible to all developers, making available a broad and sophisticated range of location based developer capabilities, APIs and tools. Location aware applications may take many forms - the ArcGIS Developer Subscription, by virtue of its 5 distinct "levels" of product inclusion, seeks to accommodate all these forms, as well as developers from varied backgrounds. If you haven't already become involved, sign-up, dive in and have a go. It's never been easier!