The previous posts all correctly point out that English is the lingua franca today, but I don't see where they have addressed how the history of the English language has lead it to become the lingua franca. I'll try my hand at this, but I don't have my usual resources on hand, so I'll have to work from memory. (I recommend the book The Story of English, based on the TV series in the link below. For a more exhaustive linguistic approach, see for example, the outer history section of the textbook by C.M. Millward.)
English was a relatively unimportant language (or, more precisely, a set of dialects) around 450 AD, when a set of Germanic tribes (mostly the Angles, Saxons, and Jutes) moved from their homelands in what is now northern Germany and Denmark to what is now England. Old English was widely spoken on the island in the centuries that followed, lost status when the French-speaking Normans conquered England, and slowly regained status in the late Middle English period. At this point, still, English was just one of many languages. It was generally less respected than Latin among scientists and theologians, for example.
It was not until the period of Early Modern English, a period also known as the Age of Discovery (c. 1500-1800 AD) that English really began to gain status. England began to establish colonies around the world and exported its language and governing systems so that it could import what it wanted, e.g. spices and rare woods and, for a while, slaves. The Early Modern English period witnessed the first time that English was spoken around the world, but still it was not nearly as widely distributed as it is today.
Today English is widely taught around the world as a second language. (The number of native speakers of English is not very large at all, compared to Spanish or Chinese, it's worth noting.) The emergence of the United States as a global power after WWII (incl. the strong economy and the enduring military presence of American soldiers in many countries around the world) took the globalization of English one step further. Still, British English is still widely taught as the prestige form in classrooms around the world.
There's a lot more to say, of course. If you ask for more (e.g. by 'message'), I'll gladly do what I can to help.