The short answer
Google reviews are the reviews people leave on your Google Business Profile — the stars next to your business in Search and Maps. Google Customer Reviews is a separate Merchant Center programme that surveys buyers after an order and feeds seller ratings into Shopping ads.
Most people asking about one mean the other. The widget shows Business Profile reviews.
Google reviews
Left voluntarily by anyone with a Google account, about a place. They drive local search, appear in Maps, and are what a customer sees when they search your name. Anyone can leave one whether or not they were a customer.
Google Customer Reviews
Opt-in, ecommerce only, and runs through Merchant Center. After an order, Google emails the buyer a survey; responses build a seller rating that can appear in Shopping ads and as a badge on your site. Tied to actual transactions, so harder to game and lower in volume.