Bitcoin is a cryptocurrency, or digital currency, created 10 years ago by Satoshi Nakamoto. There is no physical bitcoin, instead balances and transactions are stored on a public ledger (think huge database) that anyone can access. Cryptocurrency has many uses; it can be used to pay for goods, you can send it to your friends and family, trade it for your local currency or invest in it and watch the value rise over time.
Bitcoin uses distributed ledger technology (blockchain) to record transactions, people all over the world "mine" bitcoin. Bitcoin miners help to process transactions, secure the network and in return they are given a cut of newly minted bitcoin.
Bitcoin was the first cryptocurrency and has proven to be extremely popular, the amount of holders has continued to grow over time, as has the price. As awareness in blockchain technology grew, it led the way to thousands of "Alt Coins" (an alt-coin is the term given to cryptocurrencies other than bitcoin). Cryptocurrencies such as bitcoin are not controlled or overseen by any goverment or authority.
There can only ever be 21 Million bitcoin in total, with approximately 18 million tokens in existence right now, the remaining bitcoin will be mined over the next century.