A semi-colon can be used instead of a comma to separate items in a list if the list is made up of phrases rather than single words, especially if the phrases already contain commas.

for example:

I need a big bag of bananas, not too green; a tin of tomatoes, preferably Italian ones; a large box of chocolates; a two-pint carton of milk; and some grapes.

If the list consists only of short words, then a comma is used.

I need bananas, chocolates, milk and grapes.

For lists with commas we don't usually use a comma before the final item, but for lists with semi-colons it is more common (but not essential) to put one in.

There is not always a set rule for when to use semi-colons in a list. Sometimes either semi-colons or commas will work. Both of the following sentences are correct.

At the zoo we saw several different animals: black and white zebras, colourful parrots, rhinos with long horns and shy gazelles.

At the zoo we saw several different animals: black and white zebras; colourful parrots; rhinos with long horns; and shy gazelles.

When deciding whether or not to use semi-colons in a list, try using commas first and checking if the sentence makes sense that way. If it is confusing with commas, use semi-colons instead.