Both of these topics are very similar and will sometimes be used to mean the same thing. All it is? Good, free, high return marketing. Very self explanatory word-of-mouth is android app marketing that someone hears from someone else. For example John Doe gets your app, likes it, and tell Jane Doe to get it. This is free marketing for you and there is a good chance Jane Doe will at least try your app. People will believe people they know over an ad that they see.
Why is this such a high return form of marketing? If a friend tells you that you should check out an app, chances are they are going to ask you later on if you downloaded it yet. This can also be a deciding factor if you are a paid app. If Jane Doe see’s app X and it costs money, and Jane Doe knows that her friend John Doe has that app Jane Doe is likely to ask John Doe if the app is worth it.
It does not matter what your app is known for in order to make word-of-mouth marketing effective. The app Flappy Bird (recently removed from the app store and play store) was known for being a frustrating game that people only complained about. Well, people downloaded the game just to see how frustrating it was. Then those people complained, and more downloaded it.
Complete list of Android App Marketing Lessons