Many of the letters in our alphabet come from greek and phoenician letters. how does such an alphabet differ from egyptian hieroglyphics or chinese characters?

Both egyptian and chinese written languages use pictures as a basis (aka pictographs). On the other hand, our alphabet and the alphabets it stems from base their characters on sounds. 

I would say that our alphabet is different because, most of the time, individual letters do not represent words or ideas. Instead, we string them together and give them meaning based on the sequences of letters.
Chinese characters and Egyptian hieroglyphs have symbols, where each one represents something different. This would be like having every letter of our alphabet representing something, such as a car or a tree or the sun.

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