background is shorthand property for the following:
- background-color
- background-image
- background-repeat
- background-attachment
- background-position
You can detailed info on every property here
Properties order
In most of browser implementation (i think maybe older browser could present issues) the order of the properties does not matter, except for:
background-origin and background-clip: when both of this properties are present, the first one refer to -origin and the second to -clip.
Example:
background: content-box green padding-box;
Is equivalent to:
background-origin: content-box;
background-color: green;
background-clip: padding-box;
background-size must always follow background-position and the properties must be separated by /
if background-position is composed by two numbers, the first one is the horizontal value and the second the vertical value.