The := notation serves both as a declaration and as initialization.
foo := "bar"
is equivalent to
var foo = "bar"
Why not using only foo = "bar" like in any scripting language, you may ask ? Well, that’s to avoid typos.
foo = "bar"
fooo = "baz" + foo + "baz" // Oops, is fooo a new variable or did I mean 'foo' ?