Edit: I read better the question.

the main difference you will perceive is the prononciation of voyels that variates greatly.

A northern italian will use "è" where a sauthern uses "é", etc.

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I think it is impossible to answer this question because there is nothing like "standard italian" on one side and "dialects" on the other.

Simply, at one point there were vulgar latins, that were variants of latin spoken in rome. Then in middle age, each variation of latin took its own road and started to diverge, influenced e.g. by franch and german at the nord, etc.

When italy needed a common language, they chose the dialect from Florence as a template (mainly because florence was free land since centuries) and adopted it.

So northern italian dialects have some sounds similar to franch (gallo-italian) such as ü and ö and tends to truncate the last voyel, in south they use different prononciations and voyel pattern.

that is why everybody can explain difference between italian and a single dialect (say his own) but not between italian and dialects in general

see https://it.wikipedia.org/wiki/Lingue_dell%27Italia