This is the last block that was mined. On ethstats, it shows the time since something was mined.
A block is a collection of transactions. At time of writing, the last block is 3716787
Gas is a measure of computational effort. It is the internal pricing for running a transaction or contract in Ethereum.
The gas system is not very different from the use of Kw for measuring electricity home use. One difference from actual energy market is that the originator of the transaction sets the price of gas, to which the miner can or not accept, this causes an emergence of a market around gas.
This is a valid block with a chain that has the highest sum of work.
Difficulty is a measure of how difficult it is to find a hash below a given target. The difficulty dynamically adjusts so that on average one block is produced by the entire network every 12 seconds.
More information on the glossary.