Question: I have been involved in two volunteering groups for the past two years and I'm not sure If I should ever mention it in the SOP/CV.

Should I mention it in one or both my SOP and CV? Also, how detailed should I write on the type of activity/nature of the work/vision/reasoning/accomplishments, etc.

Note: I'm applying for Electrical and Computer Engineering for a master's degree.

The volunteering work is totally unrelated to the degree. The activities are focused on aiding homeless and abused animals

Answer: 
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> Should I mention it in one or both my SOP and CV? 
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Yes, in the CV (shows you're well rounded and well enough organized to keep up with a commitment outside of school), but not in the SOP (because it's unrelated) -- unless this particular experience was fundamental in motivating you to want to go to grad school.

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> Also, how detailed should I write on the type of activity/nature of the work/vision/reasoning/accomplishments, etc.
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The whole thing should fit on two (maximum three) lines.

If you have ever done any other volunteer work, you can make a whole section for Volunteer Work.

Alternatively you could include it under Work Experience, and then in the description make it clear that it was a volunteer position.
I see nothing wrong in mentioning that on your resume even if it is unrelated. If you have enough place on your 1-2 page resume, you can put it.
In my eyes, It shows that you are a passionate, social and informed person and not just a (sitting in front of the computer) geek.