What is a bitcoin app?

Could you elaborate on the concept of a Bitcoin app? I'm curious to understand how these specialized applications fit into the broader cryptocurrency landscape. Do they primarily facilitate transactions, or do they offer additional features and services? I'm also interested in knowing if these apps are available for both desktop and mobile devices, and if they require any special permissions or setup. Finally, could you discuss some of the top-rated Bitcoin apps and their unique selling points? I'm keen to gain a comprehensive understanding of this emerging technology and its applications.