Trade, mainstream, or commercial publishing is sometimes referred to as “traditional” publishing, but I don’t like the term (you’ll find out why later). Trade publishers range from huge, world-wide conglomerates with many different imprints which publish thousands of titles a year to tiny independent presses which publish one or two books each year. These publishers generally pay their writers an advance (although not all do), and royalties on every sale; the best edit each title meticulously and take great care with design and typesetting; they market and promote their books; and they employ sales staff and good distribution services to ensure that their books reach bookshop shelves, where readers will have the opportunity to browse and enjoy the books and, fingers crossed, like them enough to buy them and take them home.
The disadvantage to seeking trade publication is that it can be very difficult to find a publisher who is even willing to look at your work, let alone publish it; which often leads writers to consider other avenues.