This is only a partial answer.
Here are some differences which I noticed when looking a the linked pages in your question:
Miner Reward. Forked blocks earned the miners a full reward (5 ETH + gas fees) while uncle blocks only earned them a fraction of that
Numbers As of today (July 7, 2017, 12.25 pm), there are only 15,584 forked blocks on the blockchain compared to 310,542 uncles. For some reason (that I hope someone else can explain), forked blocks are a lot less common.
Relationship Some forked blocks include uncles and others do not. Uncle blocks by definition all include one or two uncles.
Another big difference is that, after blockchain re-organization, forked blocks are excluded from the main blockchain (per the statement on the etherscan.io page you provided) while uncles can still be included / referenced on the blockchain, and that is in fact encouraged for a number of reasons which Vitalik explains in in this article:
Stale blocks in Ethereum can be re-included into the chain as “uncles”, where they receive up to 75% of their original block reward. This mechanic was originally introduced to reduce centralization pressures, by reducing the advantage that well-connected miners have over poorly connected miners, but it also has several side benefits, one of which is that stale blocks are tracked for all time in a very easily searchable database – the blockchain itself.
Having noted all these differences, and done some web research, I still could not get to the core difference between both types of blocks (i.e. forked and uncle). I hope others will help clarify further.
Valid blocks that are not included in the main Ethereum chain because they were found a bit too late are still submitted to the network. Miners can include those blocks as "uncles" if they are not "older" (further away) than 7 blocks. They will receive a certain addition to the reward (a fraction) and the original miner of the uncle will also receive a reward which decreases with the "age" or "distance of the uncle.
It seems like the forked blocks page shows blocks that forked for any reason (e.g. and mostly because they were a bit too late) but which are NOT included as uncles later. The uncles page shows only those blocks that are actually recorded as uncles in the Ethereum chain.
While the list of uncles is complete because it can be read from the blockchain, it must be noted that the list of forked blocks depends on the connectivity of etherscan's nodes and, therefore, is not conclusive.