Buy low, sell high. EVE’s economy is almost entirely player driven which means that there are plenty of ways to make money just by playing the market.
There are two main types of trading: station trading and region trading.
Station trading involves placing buy orders and sell orders in a station to arbitrage the gap between the current highest buy price and the current lowest sell price: players sell goods to your buy order, then you list the same goods for a higher price as a sell order. You pocket the difference between the two prices in return for running the risk that you might not be able to sell something for as good a price as you bought it.
Region trading involves buying goods from a market in one region of the galaxy, then moving those goods to another region and selling them there. Because buying items from the market doesn’t automatically move them to a player’s current station, players will often pay extra for goods closer to their current location. Prices will also often vary between market hubs as supply and demand rise and fall between regions. An astute trader can take advantage of these varying prices to make ISK.
You can read more about trading in the UniWiki ‘Trading’ article.
Some people really enjoy the careful planning and execution that goes into successful trading. Traders can also make large amounts of money if they’re good at figuring out trends and staying ahead of the curve.