Bitcoin is the first cryptocurrency, it's also the most widely known one as well. On top of that, it is the highest market cap value cryptocurrency, by a fairly large margin. Altcoins are every single other coin that is not bitcoin, and there are a lot of them. Some are great, some not so great, some complete scams. See Coinmarketcap.com and check the top coins, most if not all of them are good projects with a future.