What is the equivalent platform of Ethereum Smart Contract in EOS ? And by which programming languages we can develop a smart contract? Is there a specific language like Solidity in Ethereum? And is there any demo smart contract for EOS as an example?
EOS is using a WebAssembly virtual machine to run smart contracts.
By theory you can use any language that will compile into WASM, but the most approved way is to use the C++ library and the eosiocpp tool (EDIT this will soon be replaced by the eosio-cpp tool).
Please have a look at the official developer portal.
I would also recommend to use the Docker workflow like it is described in the portal or in some questions here for less configuration and set up effort.
Also the GitHub repo gives multiple examples of possible contracts. To start learning I would recommend to use a tutorial (EDIT this will be deprecated because of V1.2) - which will also show you what additional libraries are available (like e.g. eosjs for frondent-dev or Demux for backend-dev).