This is a bitcoin block chain explorer. The block chain is a record of all the Bitcoin blocks that are discovered, where the bitcoins are stored and where they are transferred to. With it you can see where payments come from and where they go. It is not a wallet and does not allow you to actually send or receive bitcoins.
The app allows you to enter the hash/address for any block, address and transaction and view all the properties of that object. From there you can explore all the things associated with that object, and the things associated with them.
You can go Address->Transaction->Input->Address->etc Transaction->Input->Address-Transaction->etc Block->Block->etc Block->Transaction->etc as many levels deep as you like.
All the things are colour co-ordinated and all values are shown in Bitcoins rather than Satoshis. You can save your favourite addresses, transactions and blocks to the home tabs for easy access and you can also share the details about them using email, Facebook, etc.It is powered by the BlockTrail.Com API.