Some emoji apps simply offer alternative images, while others provide a new keyboard. Some emoji apps are collections of images users can send through text messages; these emoji aren't part of the recognized emoji standard.
Because Unicode offers code points for emoji, smartphones often let users change which emoji images they use by installing a different font. Users can rely on Apple's default emoji or download alternative fonts, but the difference between these options is largely cosmetic. Some of these alternative emoji fonts are based on a theme, while others are simply alternative implementations of the standard. The set of emoji characters is large, so some apps provide different ways of accessing emoji in an attempt to simplify the process; some users might prefer one of these emoji keyboards over Apple's default interface.
Some apps marketed as emoji apps actually contain images that are not part of the emoji standard. When users send an emoji image, the text message doesn't send an image itself but instead a character that tells the recipient's phone to insert the emoji image; this is why emoji often look different on the sender's and recipient's phones. These alternative emoji apps actually send the image, which uses significantly more data.