JetBrains Runtime is a runtime environment for running IntelliJ Platform based products on Windows, macOS, and Linux. JetBrains Runtime is based on OpenJDK project with some modifications. These modifications include: Subpixel Anti-Aliasing, enhanced font rendering on Linux, HiDPI support, ligatures, San Francisco font family support on macOS, some fixes for native crashes not presented in official build, and other small enhancements.
JetBrains Runtime is bundled with latest versions of all JetBrains IDEs, and is used by default, unless explicit reconfiguration was performed. The exception is 32-bit Linux systems, where IDEs require a separate JDK installation, because only 64-bit JetBrains Runtime is currently bundled. For Windows, 32-bit JetBrains Runtime is bundled, it will work on both 32-bit and 64-bit systems.
Versions of JetBrains Runtime for all supported architectures (including 32-bit Linux and 64-bit Windows systems) can be downloaded at https://bintray.com/jetbrains/intellij-jdk/.
This page has instructions on changing runtime version IDEs run under. Runtime version in use can be verified in IDE's About screen.
UPDATE Starting from 2017.1 version, 64-bit runtime is bundled on all platforms by default.