Hotels.com started as a specialist hotel search engine and has grown into one of Europe's biggest booking platforms. They're owned by Expedia, which means you're getting a big operation with decent infrastructure behind it. The actual strength here is breadth: they list hundreds of thousands of properties across Europe and beyond, and they're not trying to be clever about it. You search a destination, you see hotels, you book one.
What sets them apart slightly is the reward programme. Stay regularly and you collect points towards free nights, which actually adds up if you're someone who travels for work or takes multiple trips a year. They also push seasonal deals quite hard, particularly weekend rates at chains like Courtyard by Marriott, and they'll email you about specific destinations (Paris hotels, San Diego properties, that sort of thing). The destination guides they publish, like their Kyoto and Kobe pages, are genuinely useful rather than fluff.
The interface isn't revolutionary, but it's straightforward enough. Filter by price, by star rating, by amenity. Read reviews from other travellers. Book. You know roughly what you're getting. Customer service exists, though like most major booking sites, you'll want to check cancellation policies yourself before you commit. They take the standard payment methods, and gift cards are useful if you're buying for someone who's impossible to shop for.