The obvious is; hardneck literally have a hard neck as a result of the stem extending down to the base plate.
Soft necked varieties on the other hand do not have this stem.
Note; Due to the softer nature of the neck these can be pleated much easier when tying up for storage.
Hardneck varieties generally have to be tied individually.
Softnecked varieties are said to store longer than hardnecked varieties, however there are some hardnecked varieties that will store equally as long as softnecked varieties.
Hardneck varieties tend to have fewer but larger cloves per bulb.
Hardnecks need to be planted in late autumn for them to able to produce decent size cloves where as softnecks can be planted late winter / early spring if ground conditions allow.
If softnecked varieties are planted late winter / early spring they will not be ready for cropping until late summer/early autumn, with autumn planted hardnecks being ready a little sooner.
Hardnecked varieties produce flowerless stems with bulbils on them.
These bulbils can be potted up to increase stock!