You can't write smart contracts with Steem, but you can write decentralized apps that make use of the Steem blockchain. Steem then provides you with a blockchain that can store text and also arbitrary JSON encoded data, so there are lots of things you could built on top of it, but Steem's part in this process is limited to storing textual data.
But that textual data doesn't have to be "posts", you could link to video, pictures, software, etc. stored elsewhere, on IPFS for example. You also get the ability of Steem to then monetize these things through upvotes. Check out SteemProjects.com, that website lists hundreds of apps written on top of the Steem blockchain, that's the easiest way to get an idea of what is possible in this regard.
RE: Steem Dapp Development