If you read something aloud you use your voice, not just your eyes. You might feel shy reading your poems aloud in front of strangers.
Aloud used to mean "very loudly," but now we use it to mean "speaking voice" as opposed to the silent voice of our thoughts. You might have to read something aloud to your grandmother if she's having trouble with her eyes. If you accidentally said something that you meant to keep to yourself, you were thinking aloud. If something is terribly painful, you might cry aloud — unable to contain yourself.