Both the Summer Olympics and World Cup are the two biggest worldwide events that occur in the world of sports. Both also share some similarities:
-Played in the Summer (duh)
-Played every 4 years
-Many countries are involved
-Many never before seen athletes make their name
However, when looking at the question at hand, 'Which event means more to the world?' there's only one right answer. And it's not even close. The Summer Olympics, by far, mean more to the world.
It includes many different sports, and isn't just focused on one sport.
While the World Cup features the most popular sport nationally, not every country is soccer crazy. Some don't have enough athletes to support team sports like that, so they might not be as emotionally invested. Plus, the Olympics had a 16 team soccer tournament anyways to crown a medal winner. It's not like it's included out of the games.
Many countries can achieve some type of success.
There's many events to achieve medals in, so that gives many smaller countries reason to watch the games. In the World Cup only 3 countries (out of 32 total) get rewarded with a medal. In the Olympics, there's 302 chances for Gold medals ALONE, not including Silver and Bronze. Sure, there's going to be some events that people don't remember, but if you see a fellow countrymen in an event, there's a higher chance that you're gonna be interested in watching it.
More countries are involved.
If more countries are involved, there's a greater chance of people around the world caring for it. Here's a map of all of the countries involved in the 08 Olympics:
(Taken from 08 Olympics results)
Yeah, pretty much 99.9% of the entire world is involved in the Olympics in at least one sport. Which, in turns, provides more value and importance to the world since their country is represented.
(Taken from 2010 World Cup; Note: Blue = participated. Yellow = DNQ)
You look here, the World cup was heavily dominated by European and South American countries. Why should countries from Asia or anyone from Africa (except South Africa, who was only there because they were the host country) even bother to watch when their country isn't represented? Especially since they don't have a variety of sports to choose from. There's soccer, and that's it.
It's been around longer.
The Olympics first started in 1896, giving them over 30 years of history on the World Cup, which started in 1930. It has more prestige, memorable moments, and relevance to the world because of the large amount of countries involved.
While people may argue that the 2010 World Cup had roughly 26 billion people viewing it to only 4.7 billion for the 08 Olympics, they should consider the fact that the coverage worldwide for the Olympics wasn't as easily accessible and that in the past two years technology has improved where watching matches on your phone/computer isn't as difficult as it once was, which = more viewers.