You use some to refer to a quantity of something or to a number of people or things, when you are not stating the quantity or number precisely. 一些
Robin opened some champagne.
He went to fetch some books.
Several is used to refer to a number of people or things that is not large but is greater than two. 几; 一些
I had lived two doors away from this family for several years.
Several blue plastic boxes under the window were filled with CDs.
Several is three or more; some can imply any number, usually more than one.