Declarations are another kind of statement. A declaration is like a promise that a certain variable of a given type will exist, eventually. In some cases, this promise can be enforced by the interpreter, like when not initializing a class's declared field, or when assigning the wrong type to some variable. But this is mostly a feature intended to make the life of a (future) compiler easier.
x:int;
... this declares a variable called "x" of type int.
x = 'hello world!';
... this erroneous assignment will throw an exception.
x = 1;
... this assignment is accepted.
Function declaration has a special syntax of its own, also known as "signature":
f:\x:int,y:int:int
... this declares a function that takes two ints (x and y) and returns another int.
You can declare a list of any given type like so:
x:int[];
// x = 1; // wrong!
x = [1,2,3] //ok!