Difficulty is a measure of how difficult it is to find a hash below a given target.

Target is a 256-bit number (extremely large) that all Bitcoin clients share. The SHA-256 hash of a block's header must be lower than or equal to the current target for the block to be accepted by the network.

Target = current difficulty = output by Bitcoin's getDifficulty RPC call (Difficulty is basically a different representation of the target to make it easier for normal humans to understand it.)

Hashrate is computed hashes per second.

difficulty = hashrate / (2^256 / max_target / intended_time_per_block)
             = hashrate / (2^256 / (2^208*65535) / 600)
             = hashrate / (2^48 / 65535 / 600)
             = hashrate / 7158388.055

Sources:Relationship between Hash-Rate and Difficulty && https://en.bitcoin.it/wiki/Difficulty