A base currency is the first currency that appears in a trading pair. These currencies are always quoted in relation to another because you are buying one while selling the other. The base currency will always appear first and will be followed by the second currency, known as the quote, or counter, currency.
The quote currency is the second currency listed in the trading pair. It is also known as the counter currency. The price of a pair reflects how much it costs to purchase one unit of the base currency by selling the quote currency.
For example: Let's look at Bitcoin/Litecoin (BTC/LTC) and Ethereum/Bitcoin Cash (ETH/BCH). In these instances, BTC and ETH are the base currencies while LTC and BCH are the quote currencies.