A trademark is never protected for "everything"—it is protected for the specific goods and services a business actually sells, and the USPTO files all of commerce into exactly 45 numbered categories borrowed from the international Nice Classification. This guide explains what those 45 classes are, why two companies can lawfully own the identical word in different classes, and how class choices silently set the cost, scope, and conflict profile of a registration. It walks the USPTO Trademark ID Manual, decodes the January 2025 fee overhaul that scrapped the old TEAS tiers, and shows why "coordinated" and related classes can sink an application even when the class numbers differ. It pins down a subtle but decisive point of law—that classification is "strictly ministerial" and does not define legal rights, even as it drives fees and search obligations. A running hypothetical follows an invented candle company from one class to six, and the piece closes with a classification-error field guide, an FAQ, and links across the UpLaw trademark library.