+888.555.1212
Plus (+) is usually a prefix for a country code, to let you know that the country code is included in the number. That's because each country has its own way of accessing the international network. So "+" means "dial whatever prefix is necessary to place an international call."
Alexander Graham Bell was American, so the United States gets the Country Code of "1".
+1.888.555.1212 let's people know that you're an American and can be reached at that number. It's transparent to most of us because we also use a "1" for long-distance calls.
The Country Code for Germany is 49. When I lived in Nuernburg, my city code was 911, so my folks had to dial 011-49.911.XXXXX to reach me. I dialed 001-888-555-xxxx to reach them. They were always baffled that they had to dial the emergency number (in the United States, you dial 911 to reach the police or an ambulance).