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They are very close, and could both be used in similar contexts, depending on what you want to emphasize.
По- simply states that someone has gone off somewhere; we may not know or care whether they've reached their destination (though the destination is usually mentioned, unless it's clear from context).
-Она пошла в буфет. = 'She went to the snack bar.'
With у- the focus is on absence, the place where the person no longer is, so the destination may not be mentioned; saying someone is 'gone' in English expresses a similar nuance.
-Она ушла. = 'She left./She's gone.' (We don't expect her to be back soon.)
Шёл/шла/шло/шли are past tense forms of the imperfective идти; they can often be translated "was on the way (somewhere)," and often sound incomplete without some further context.
Когда мы шли по улице, все кричали: «Смотрите, русские идут!»
-When we were walking down the street, everyone was shouting "Look, the Russians are coming!"
They can also focus on the action itself in process:
-They were walking very slowly.
Пошёл/пошла etc., on the other hand, often translate as 'set out.'
-She set out for home/went home/headed for home.
Идти and ехать are often used to describe a single instance of going somewhere, and this may be the most common context. Repeated trips generally involve going in more than one direction – there and back – which calls for multidirectional verbs like ходить and ездить.
Remember that the basic meaning of идти and ехать is going in one direction (that's why they're called "unidirectional"). There may be contexts when, even though you're making a round trip, your attention is focused only on the trip in one direction.
Logically, there must be round trips here – surely you have to leave in order to come home again! But when we talk about something that happens only on the way home, it makes sense to use идти or ехать – because it's only one direction that's relevant.
So, идти and ехать can be used for repeated actions, but only when the context makes clear you're talking about one direction only.
For another example, see this blog post.