Bitcoin is electronic cash that operates on a blockchain. A blockchain is an array-like data structure composed of blocks, and a block is like an array made of individual transactions (like individual entries in an accounting ledger).

Blocks are created approximately every 10 minutes, and each new block references a previous block with an identifier which is a hash (which can be thought of like a unique fingerprint). The hash of a block is uniquely determined based on all the transactions is has within it, combined with the hash of the previous block. So changing any transaction within a block or the previous block would change the hash of a block.

Each transaction within a block is composed of one or more inputs and one or more outputs. A transaction can have 1 input and several outputs, or several inputs and 1 output, or 1 input and 1 output, or many inputs and many outputs –

This describes the basic structure of the bitcoin blockchain, and how the chain grows, over time.

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Transactions are an important aspect of the blockchain, and I will describe a bit about how transactions work and how a transaction is created.

First, we must understand keypairs; which is a pair of cryptographic keys; a Private Key and a Public key – thus, a pair. It is okay to call these a "pair of keys" as well, but it very important to understand that one key is Private and should not be shared with others, while one key is Public and is okay to share with others, because it is commonly used to generate an address for which other people reference in their transactions, when they send bitcoin to you. Similar to how a postal address or an email address is unique to you at a point in time, a bitcoin address can be used to send you things; in bitcoin's case, bitcoin transactions can be sent to you.

Generating a keypair is important to understand.

A mnemonic, which is commonly a 12 word phrase, or sometimes a 24 word phrase can be used to reliably generate a seed, which is a random number, which can then be used to generated keypairs.

An example of a 12 word mnemonic is:

retire hammer table chalk horse sweet outdoor eggs grant pact funny furnace

Another example is:

play notable vague jungle police jump eyebrow sound wet reflect melody filter

These words can be generated at random by software. And it is VERY important to remember the mnemonic phrases you use, and not to share them, because the phrases are used to generate keys. And somebody else with your phrase could generate your keys and gain the ability to access or move bitcoins associated with those keys.