Mollusks are animals like mussels, clams, snails and slugs. Cephalopods like octopus and squid are also mollusks. The name Mollusk means "soft", because mollusks don't have any skeletons. The first mollusks probably evolved from early segmented worms around 500 million years ago. At that time, all animals except arthropods still lived in the ocean, and all these early mollusks also lived in the ocean.
But mollusks also have some things that segmented worms don't have. Many mollusks have shells made of calcium to protect them.
Most mollusks still live in the ocean, but a few kinds of snails and slugs have evolved to live on land. Snails have probably evolved to live on land not just once, but on several different occasions, independently. The earliest evidence of land snails comes from about 325 million years ago, around the same time that frogs came out of the water, and so frogs love to eat snails. But there weren't so many land snails and slugs until about 145 million years ago, in the time of the last dinosaurs and the first birds - and birds also love to eat snails.
To find out more about slugs and snails, check out these books from Amazon.com or from your library: