The precise definition of each statistic and graph displayed in
https://ethstats.net.

Best Block is the heaviest block regarding the cummultative difficulty, or in simple words: the highest block number of the longest valid chain.
Uncles are orphaned blocks, but in oposite to other blockchain systems, uncles are rewarded and included in the blockchain. Shows current bloc's uncle count and uncle count of last 50 blocks.
Last Block shows the time since the last block was mined, usually in seconds.
Average Block Time is, well, the average time between two blocks, excluding uncles, in seconds. Should be something around 15 seconds.
Average Network Hashrate is the number of hashes bruteforced by the network miners to find a new block. 5 TH/s means the network power is at five trillion hashes per second.
Difficulty is the current mining difficulty to find a new block which basicly means how hard it is to find a matching hash.
Active Nodes is the number of connected nodes to the Ethstats dashboard, (not the whole ethereum network!)
Gas Price is the price miners accept for gas. While gas is used to calculate fees. 20 gwei is the current default, which stands for 20 Giga-Wei which are twenty billion wei that is 0.00000002 ETH.
Gas Limit is the block gas limit. It defaults to 1.5 pi million gas (4,712,388) and miner can only include transactions until the gas limit is met (and the block is full). The gas limit is the analogy to bitcoin's block size limit, but not fixed in size.
Page Latency and Uptime are specific stats for the dashboard.
Block Time Chart shows the actual time between the last blocks.
Difficulty Chart shows the actual difficulty of the last blocks.
Block Propagation Chart shows how fast blocks are shared among the nodes connected to the dashboard.
Last Block Miners are the public keys of the miners who found most of the last blocks.
Uncle Count Chart shows numbers of uncles per 25 blocks per bar.
Transactions Chart shows numbers of transactions included in last blocks.
Gas Spending Chart shows how much gas was spent on transactions in each block, note the correlation to the transactions chart.
Gas Limit Chart shows the dynamicly adjusted block gas limit for each block.