Bitcoin is a cryptocurrency that was first introduced, then considered a pioneer of cryptocurrency. Therefore, bitcoin can be called the center of all coins even though technically each coin has a different algorithm.

Altcoin is an alternative coin, which you can use to cross-transaction to bitcoin. For example, BTC / ETH, BTC / LTC, and others. Basically, altcoins are also similar to bitcoin, some can be mined, some cannot be mined.