To buy a coin, we usually do it with Bitcoins. For example, if we want to buy XRP, we have to pay with BTC. In this case, we are active in a Bitcoin based market. However, on many platforms, we can buy the currency not only with Bitcoin, but also with an alternative one. The most common coins used for this purpose are Litecoin (LTC), Doge, Ethereum (ETH), Monero (XMR), USDT (Tether) and others.
There is a minimum trade value in these markets also, but it is usually lower than what is in Bitcoin-based markets. So, if we have a coin whose amount is small to be traded into Bitcoin-based markets, we can eventually trade it to an altcoin-based market.
This variety of markets creates many profit opportunities, since we can use BTC to buy a coin, sell it for later in a LTC market for example, and then sell the LTC and get the BTC, a method known as "Triangular Arbitrage ".
It is worth noting that due to the many markets available and because of the many exchanges, more complex shapes can be made.
