android.text.style.ReplacementSpan class is a supper span class which you can extend to create your customized span class. You can draw anything as you want in your custom span class, and then use it to replace the specified characters in your textview string.
android.text.SpannableString is a character sequence type class. You can use it as a String object in android TextView. But SpannableString can make TextView content more colorful and diverse. This article will show you examples about how to use SpannableString with TextView.