Winter pansies used to be a strain called Great Universal - these were said to be slightly hardier than other varieties. Since then though, its been discovered that the little viola types are a bit hardier and more likely to flower in winter, although the truth is, in dark days with cold temperatures, they all stop flowering, and start again in spring. By the end of spring, they're usually leggy and need replacing, but sometimes cutting back will mean another show of flowers later. So, in theory, your summer pansies are newer plants, less likely to go to seed, and will have some useful weeks ahead of them in terms of flowering. Pansies bought last winter probably look fabulous right now,but will, as I say, get leggy and seed, usually by June.

Not a lot really. It's down to what the garden centres want to call them and the time of year they were sown. As mentioned about, Pansies will not flower under 5 degrees C whatever the varieties.

yep biologically speaking they are the same plant. But brought into flower early autumn, winter pansies, allowed to flower spring onwards, summer pansies.
trim/dead head spent flowers and they will keep going for ages. I often take the stems off with shears now, just to stmulate new grotwh.