This should be the last one. So it's gonna be longer then the first so i don't have to brake it up into more parts.
 What is a block?
A block contains some data, the hash of the block,and the hash of the previous block. So for this example we are gonna use the bitcoin block chain. The data stored in this block would be the data about the sender, the reserve, and the amount of coins. (Side note this is why i said in a previous post that it isn't safe for dark net transactions.)
A hash for a block.
A hash is like a digital finger print. It identifies a block and its content. It's always unique to that block. Once a block is created the hash is being calculated. Changing something inside the block will cause the hash to change. Once a hash changes on a block, it is no longer the same block.
The 3rd part of each block is the previous hash. This is what makes a chain of blocks and makes it so secure.
Heres a bit of a visual. 
Block 1.                 Block 2.             Block 3.
Hash: 1F9Z          Hash: 5DS4      Hash: 8I6P
P.hash:0000        P.hash: 1F9Z    P.hash: 5DS4
 
So as i have shown block 3 points to block 2, and block 2 points to block 1. Block one doesn't point to anywhere because it is the first. Making it special. This is called the genesis block. It's you tamper with say the hashes in block 2 every block that comes after it would be void. So they combat this with a system called proof of work.
Proof of work. 
 This mechanism slows down the creation of new blocks. In bitcoins case about 10 minutes to calculate the proof of work and create a new block. If you tamper with one block you have to recalculate the proof of work for all the other blocks. So the security of a blockchain comes from the use of having and the proof of work systems.
 The second way it is secured is by distributing. Instead of having one person or group to manage the chain it uses a p2p or peer to peer network that anyone can join. When someone joins the network he/she gets a full copy of the blockchain. The node can use this to make sure everything is still in order. When a new block is created it is sent to everyone in the network,and it is verified by each node in the network to make sure it hasn't been tampered with. If it hasn't been tampered with then each node adds this to their blockchain. At this point all the nodes agree on which blocks are valid and which ones are not. Blocks that are tampered with are rejected by all the nodes on the network.So to successfully tamper with a block chain is next to impossible. I hold you like this. 
CHEERS.