If the sum of the coinbase transaction's outputs must be less than or equal to the current reward (25 btc currently), why in practice do blocks have a slightly higher coinbase transaction output (for example this block has a coinbase transaction with an output value of 25.25058085 BTC.
The extra 0.25058085 BTC is the sum of the transaction fees on the block (each transactions fee is the difference between the value of the transaction's inputs and the transaction's outputs).
As you can see, this block which had no transactions on it, and therefore no transaction fees, had a coinbase transaction whose sum of outputs was exactly equal to the reward (50 btc at the time).
In addition to the reward, the miner may spend all transaction fees to himself when he discovers the block.
Transaction fees are what's left of a transaction's input after subtracting the outputs.
Input Output 1 BTC --------------> 0.99 BTC ---> Transaction Fee = 0.01 BTC
The sum of all transaction outputs on the whole block must be smaller or equal than the sum of all transaction inputs on the whole block plus reward.