Bitcoin is the name of the cryptocurrency, and you aren't limited to just buying it in increments of 1 full Bitcoin as the currency is divisible, just like dollars. A satoshi is the smallest unit of bitcoin.

The cryptocurrency can be split in to Satoshi's which are worth 1/100,000,00 of a Bitcoin, or 0.00000001 BTC.

An easy way to think of this is similar to dollars and cents, the Canadian dollar is equal to 1 CAD and a Canadian penny or one cent is worth 0.01 CAD. If the value of 1 BTC is equal to $40,000 CAD then 1 Satoshi would be equal to $40,000 X 0.00000001, or 0.0004$

Also, for investing in Bitcoin, it is very convenient to not have to buy a full Bitcoin if you are only a casual investor or are risk averse. 

Interestingly, the Satoshi is named after the founder of Bitcoin - Satoshi Nakamoto.