Two different United-branded lounges sit inside many of the same airports, and travelers regularly confuse them. Here's the honest distinction.
United Club is the airline's general-purpose lounge brand, accessible through annual membership ($650), the United Club Infinite Card (Chase, $525 annual fee), or a single-visit pass ($59). United Club locations exist at all of United's hubs — ORD, IAH, EWR, IAD, DEN, SFO, LAX — and at many spoke airports as well. The lounge product is solid but unspectacular: complimentary cocktails, light food, Wi-Fi, comfortable seating, business amenities.
United Polaris Lounge is an entirely different and significantly more premium product. Polaris Lounges exist only at ORD, EWR, IAD, IAH, SFO, and LAX, and access is restricted to passengers ticketed in international Polaris business class on a same-day United-operated or qualifying Star Alliance partner flight. There is no membership, no credit card, no annual fee that grants access. You either have a qualifying international business-class ticket or you don't.
The Polaris product is genuinely premium: dedicated dining room with full table service, made-to-order entrees, daybeds and shower suites, full bar with premium liquor, and dramatically lower crowding than the adjacent United Clubs. The Polaris Lounge at SFO and the rebuilt one at EWR are widely regarded as among the best business-class lounges in the United States.
A practical confusion: at ORD, IAH, and EWR, the Polaris Lounge and United Club are within the same general area, and confused travelers regularly try to enter the Polaris Lounge with United Club access. Polaris staff will turn you away. Conversely, qualifying Polaris passengers can use United Club instead — but rarely want to once they've seen the Polaris product.
Verdict: if your itinerary includes any same-day international Polaris business segment, use the Polaris Lounge. Otherwise, use the best United Club available at your terminal, ideally one of the renovated locations like the polaris-adjacent United Clubs at SFO or the new T2 location at LAX.