以下摘自The Go Programming Language：

A var declaration creates a variable of a particular type, attaches a name to it, and sets its initial value. Each declaration has the general form
var name type = expression
Either the type or the = expression part may be omitted, but not both. If the type is omitted, it is determined by the initializer expression. If the expression is omitted, the initial value is the zero value for the type, which is 0 for numbers, false for booleans, “” for strings, and nil for interfaces and reference types (slice, pointer, map, channel, function). The zero value of an aggregate type like an array or a struct has the zero value of all of its elements or fields.

short variable declarations are used to declare and initialize the majority of local variables. A var declaration tends to be reserved for local variables that need an explicit type that differs from that of the initializer expression, or for when the variable will be assigned a value later and its initial value is unimportant:
i := 100 // an int
var boiling float64 = 100 // a float64
var names []string
var err error
var p Point