do they have any symbol or logo?
Yes, the Olympic rings which consists of five intertwined rings .
Olympic Games
. The Olympic symbol, better known as the Olympic rings, consists of five intertwined rings and represents the unity of the five inhabited continents (Africa, the Americas (when considered one continent), Asia, Europe, and Oceania). 
how does an athlete get to participate in the olympic games All professional athletes are eligible for the Olympics, subject to the approval of the IFs. what are these games? he modern Olympic Games or Olympics are leading international sporting events featuring summer and winter sports competitions. how different is it from the ancient games? In the ancient games, the competition was among representatives of several city-states and kingdoms of Ancient Greece. Whereas the modern games are international nature.
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Olympic Games [SEP] Symbols
The Olympic Movement uses symbols to represent the ideals embodied in the Olympic Charter. The Olympic symbol, better known as the Olympic rings, consists of five intertwined rings and represents the unity of the five inhabited continents (Africa, the Americas (when considered one continent), Asia, Europe, and Oceania). The coloured version of the rings—blue, yellow, black, green, and red—over a white field forms the Olympic flag. These colours were chosen because every nation had at least one of them on its national flag. The flag was adopted in 1914 but flown for the first time only at the 1920 Summer Olympics in Antwerp, Belgium.