Question

how is ginger different from onion

Answer 1

Both are modified underground stems but rhizomes lie horizontally. Horizontal rhizome stems have leaf nodes or ‘eyes’ that can sprout new stems and leaf buds for future asexual growth. The eye is where the plant has stem cells that are able to differentiate into new tissue for growth. 

Bulbs are abbreviated stems with surrounding layers of modified, succulent, leaves that stockpile nutrients. The layered onion bulb has the single bud that will produce a new stem when they break dormancy.
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Answer 2

- colour ; 
- taste ; 
- dishes to be used in ; 
- can make ginger tea, not onion tea; also yummy onion soup but not ginger soup ; 
- they belong to different plant families ; 
- their country / area of origin is different ; 
- their microelements / vitamins / etc. contents are different ; 
- their medical use is different (see asian medicine for specifications, especially for ginger) ; 

!!! - the onion bulb is basically an engorged stem, depositing nutrients (NOT a root, as some ppl think) ; 1 onion bulb means 1 plant ; the ginger rhizome can be divided and replanted individually resulting in more plants (same as ex: Canna Indica , a decorative garden flower plant) - !!! :)