The shorter answer: Nothing. Different ingredients or preparation methods can be used to make various types of pasta/macaroni, but generally, the words are interchangeable for the same food product. It can be fresh or dried.
The longer answer: There is evidence in ancient times to suggest the people of the Italian peninsula were making and eating pasta/macaroni, although the dishes were called by other names, such as lagana for an early Roman type of pasta dish. The earliest mention of maccheroni being produced is in a 1509 edict in Naples. A few centuries ago, a derogatory nickname for Neapolitans and Sicilians was “macaroni eaters.” Macaroni is the word Italian Americans, who are primarily Southern Italian from Naples and Sicily, used for the general food. Macaroni is also known by type, such as spaghetti, fusilli, rigatoni, tagliatelle, etc. Later, in the 1970s & 1980s, when Italian restaurants serving “Northern Italian” cuisine became popular, and chefs from Northern Italian cities gained airtime in the media, the term pasta became the preferred term amongst those in media. For a more detailed history of the origins of pasta/macaroni, please see my book, Authentic Italian.
The more detailed answer: Italians use both words pasta and macaroni for different preparations of the food. If the dish is served with a sauce/gravy (another Italian-American debate), then it is called macaroni. If it is served in a more soupy context, then it is pasta. For example, pasta e fagioli or pasta fazool or bast e fazool, in Neapolitan, is a soup made with short pasta. In Neapolitan, “p” has a “b” sound and the final vowel is left off. This is why we say we are eating “bast and peas,” or “bast and cauliflower,” or “bast and ciceri (chickpeas),” or “bast and beans.” These dishes have a more soupy quality to them, with a more liquidy sauce. Every child with an Italian grandmother remembers eating pastina, which translates to “little pasta,” a more or less liquidy dish made with very tiny pasta and butter or also with cheese and/or eggs. It’s called PASTina because it’s soupy. If it had been a sauce/gravy served on top, maybe it would have been called macarina (isn’t that a dance?!). For macaroni, if we are eating a particular macaroni dish, we use the name of that macaroni, as in, “Tonight, let’s have fusilli with ricotta,” or “What about spaghetti with broccoli sauce,” or “How about rigatoni and gravy?” So to summarize, while technically, it is the same thing, whether or not it’s pasta or macaroni depends on its preparation.
–Dina M. Di Maio, author of Authentic Italian: The Real Story of Italy’s Food and Its People