Sticker glue is known as a “pressure sensitive adhesive”, because it forms a bond when you stick the decal onto a clean surface and press down on it. The pressure helps improve the bonding.
Sticker removers are combinations of chemicals that are designed to soften up or melt the adhesive, and cause it to become less tacky.
PSAs are typically made of rubbers, acrylics (plastics), or silicones, combined with a “tackifier”. The tackifier is usually a resin solvent, or a special silicone solvent.
Adhesive removers are mixtures of mineral spirits, propane, citrus extract (oils), alcohol, freon, naphtha, and water. They’re designed to melt into the adhesive, but not damage the surface the sticker is attached to, and also to evaporate.
Safety
Adhesive removers are not safe. Look at that list: most of the stuff isn’t safe for you. So, wear gloves, or apply them with a little swab or stick. I’ve personally used my fingers, but no more. Sheesh.