The id
is just a unique identifier for the column. You can set it to anything you want. It's only use is to provide an identifier when you want to refer to your columns from code.
The field
specifies how the column binds to the underlying data. Suppose your data looks like this:
data = [
{ firstName: "John", lastName: "Smith" },
{ firstName: "Fred", lastName: "Jones" }
];
When you define the column you can tell it which value you want to display from your data array.
columns.push({ id: "anythingyoulike", name: "Surname", field: "lastName", width: 40 });