I don't see a definition of subspecies or a definition of variety that would separate it from subspecies in Ronalawn's link.

A simple answer is that subspecies is a broader division than variety. In other words, there could be multiple varieties of one subspecies. In actuality, there is considerable disagreement about which should be used when, and it's quite common to find a name referred to as both a subspecies name and a varietal name.

I'm not aware of any formal rule for this, but from my experience, subspecies classification is often given to a group of plants that are geographically separated from a similar but slightly genetically different group of plants of the same species. Quercus ilex subsp. ilex and Quercus ilex subsp. rotundifolia are examples of this. Varietal classification is often given to a group of plants living among (there's a word for that, that I can't remember right now) a similar but slightly genetically different group of plants of the same species. Gleditsia triacanthos var. inermis is a good example of this. Variety differs from form/forma in that a variety implies predictable inheritable traits where form/forma traits occur sporadically within a population.

P.S.

Don't confuse cultivar with variety (or "cultivated variety"). The origin of cultivar may be based on the word variety, but, botanically speaking, the word cultivar is NOT synonymous with the word variety. This gets a little more complicated, but some types of cultivars (yes, there are multiple types) are more similar to botanical varieties than others. Many types of cultivars have absolutely nothing in common with varieties or, if such existed, man-made varieties.

A variety belongs to same species but expresses itself differently in different altitudes (mostly plants). A subspecies is a newly given name to the members of of a species who have been geographically distanced and have some genetic and morphological differences. They can still interbreed but normally do not.

"A variety belongs to same species but expresses itself differently in different altitudes."

I don't think altitude has anything whatsoever to do with variety, at least in the vast majority of cases.

"A subspecies is a newly given name..."

I don't understand why you think "newly given" is relevant.

"They can still interbreed but normally do not."

The only two factors I can think of right off the top of my head that would keep subspecies or varieties from interbreeding is geographical remoteness or incompatible bloom time. Other than that, I would think they would interbreed equally as well as members of the same subspecies or variety. Maybe that's what you were trying to imply, but I'm not sure.

This is a good question. Some examples of subspecies are the different flowering cherries originally native in the wild, or the difference between Black cherry and Capulin cherry. Another example is the difference between Western Maidenhair fern and Eastern Maidenhair fern.

A subspecies can freely interbreed and produce fertile offspring, yet it has developed particular differences to be best adapted to its climate, ecological niche, or sometimes sexual mating strategy. In some cases crosses between different subspecies may be slightly less hardy than their parents, or more poorly suited to success for some reason.

Some people might consider many of the original wild rose types as subspecies, because each was substantially well adapted to different growing conditions and had very noticeably different growth behavior.

In many instances there is often not a clear line between species, subspecies, and varieties, but each classification level tends to have different connotations. If the differences manifest purely as vissible traits, different fruit qualities, it is probably just varieties of the same species.

And then there is ''type''. I see it used a bit. I gather it is not part of any taxonomical hierarchy. Is that correct?

There are also sub- tribes, tribes,sub-families,and families.

Somehow overlooked, the link in the first reply to: Classification of Plants - is good, I mean like really good. Check it out.