In Http Get Method all the values that user sends to the server, embedded with the URL that we send as the request, where a third party can easily watch it. If I say it more technical way, by using the HTTP GET method, we are sending user details in the header part of the request not in the body.
Where as in Http Post method we transfer confidential data to a different location by using HTTP. The reason is the content goes inside the body, not with the header as in GET method.
You can also refer android developers website
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solved Difference between httppost and httpget method in android? [duplicate]