椅子 is a chair 凳子 is a stool A chair has a back; a stool doesn't. Sometimes Chinese people may say "凳子", and then bring a chair to you, but what they mean by saying "凳子" is just referring to something to sit on.

区别在于有没有靠背

地域问题，像我们这边很少人说凳子，几乎说的都是椅子。

通常来说，区别就在于长的不一样。。椅子有个靠背，凳子没有，而且稍小。

there is no clear diffrence between thses two things.generally "凳子"is smaller than "椅子" 这两种说法差别不是很大，在中国的不同地域叫法不同，你可以先这么理解，凳子没有靠背，比较小，而椅子有靠背，比较大