Difference between RxJava and RxAndroid?

RxAndroid does not replace rxjava. It is a small library of Android specific rxjava utilities. For example it makes the Android application main thread available as an rxjava scheduler.

The major difference between RxAndroid and RxJava is Schedulers. Schedulers are specifically available in RxAndroid not in RxJava which helps in efficient threading.

https://www.androidhive.info/RxJava/android-getting-started-with-reactive-programming/

RxAndroid can be understood as an extension to RxJava, that helps you to use it on Android platform easily. See the description on RxAndroid Github repository:

Android specific bindings for RxJava 2.

This module adds the minimum classes to RxJava that make writing reactive components in Android applications easy and hassle-free. More specifically, it provides a Scheduler that schedules on the main thread or any given Looper.

Note that when you're using RxAndroid, you should also use the latest RxJava version as a dependency:

Because RxAndroid releases are few and far between, it is recommended you also explicitly depend on RxJava's latest version for bug fixes and new features.