Creatures and Monsters are enemies in Witcher 3: Wild Hunt that are not clearly defined. Some do it via the intent of the being while others do it by such a being's nature. Witchers see monsters as things they must slay, often those that react to silver negatively are considered monsters. As well creatures of magical or dark origin are also normally considered to be monsters. Some monsters are welcomed by humans and others due to their benefits, such as trolls whose ability to build and repair things make them valuable additions to rural societies.
The Witcher's code roughly categorizes monsters as creatures of no sentience who are deemed a threat to human society. Because of that, a monster slayer has to attempt to reason with an opposing creature that bears any form of intelligence before he attempts to remove it. It's not unheard of a Witcher resolving issues with aggressive creatures by reasoning with them, nor is it rare for them to engage in mortal combat with entities that exhibited advanced intelligence simply because the parties in question where malicious and threatened human lives. Geralt himself was once asked to verify - during the events of the first game - if it was true that the Silver Sword was for slaying monsters and the Steel Sword for killing humans. He replied "they are both for monsters".
You can find a list of monster drops in the Monster Remains page, to assist with your crafting.
See Blood and Wine Monsters for those included in the expansion.
Enemies in the Witcher 3: Wild Hunt can be broken down into distinct Classes. These help to further discern differences and similarities between Creatures and Monsters. The following is a list of Class "Types":
Devil by the Well
Hound of the Wild Hunt
Jenny O' the Woods
King of Wolves
The Apiarian Phantom