What comes standard varies by country and company, so the honest baseline is this: nearly all rentals include the legally required third-party liability for the country, covering damage you cause to others, and most quotes include a collision damage waiver and theft protection, but with an excess or deductible that can be substantial, meaning you still pay the first chunk of any damage or theft yourself. The cheap headline rates you see on comparison sites are usually built on exactly that high-excess structure.
Your decision is really about the excess: you can reduce it with the counter's zero-excess upsell, cheaper standalone excess insurance bought before the trip, or coverage through certain credit cards if the terms fit.
Whatever you choose, check the exclusions, since tires, glass, undercarriage, roof, and gravel roads are classically excluded from standard CDW, and driving across borders or by unlisted drivers can void everything.