Bitcoin is a decentralized peer-to-peer cryptocurrency. What peer to peer means is that each node on the bitcoin network is running a bitcoin software that enforces rules based on the open source software that can be examined by anyone in the world on the github software repository. The node also broadcasts transactions (this is how a bitcoin is transferred from one bitcoin wallet address to another by the sender broadcasting a transaction from his bitcoin wallet that is signed by the private key belonging to that source bitcoin address, and nodes pick it it up, broadcasts it to other nodes, and propagates across the network). Confused yet?
If you ever download on bittorrent, it's similar tech.
To answer your question, this is not "valid" currency if you mean like central banks or governments enforce anything. Closest one is Japan has put up a law recognizing it as a currency but they cannot enforce anything on the bitcoin network. This is the strength of the bitcoin network and blockchain technology being it's decentralized. A bitcoin has no value except what the market's perception of its value. It used to be worth fraction of a US penny, now it's worth $2,600 per bitcoin. You'll always hear of the 2 pizzas that were bought for 10,000 bitcoins. Those bitcoins are still the same bitcoins on the network and each one is worth $2,600, but let's say 50 of those 10,000 bitcoins were sent to an address belonging to John Smith, and John had a heart attack before telling anyone or passing it on, the 50 bitcoins are "lost forever" as no one can use them as the private key is gone. Satoshi mined about 1 Million bitcoins (worth $2.6B) but either he never meant to profit from them as those coins have not moved from the addresses they were sent (back then the reward for the miner was 50 bitcoins, and you could mine with a lowly laptop). Satoshi's ideals railing against the central banks may also be a reason and he may have destroyed the private keys to those bitcoin wallets/addresses. So, this is where the deflationary nature of bitcoin comes from as many of the 21 Million bitcoins (after they are all mined) may be lost, meaning there may really be 18 Million that are in use.
Blockchain is simply a database containing all the bitcoins on the network and their history from the very time of its creation and where they were transferred from what address to the next. A question that comes up is what if a powerful state nation wanted to create 1 Million bitcoins? The 7,391 nodes (one of them is mine) will simply ignore the broadcast on the bitcoin network. If the powerful nation put up enough nodes, say 10,000, that will accept the transaction creation of 1 Million bitcoins, then it would be running on a forked network that is not the bitcoin network everyone is trading the $2,600 per bitcoin network. Altcoin network.
Only 12.5 bitcoins can be created every ~10 minutes as reward to the miner that mines the block. The reward will get cut to 6.25 in the future, then cut in half again in the future and so on.
Nodes on the network:
https://bitnodes.21.co/
