A manufacturer's datasheet is the primary source of info for most parts. Physical measurements like length, width, height, pitch are almost always listed along with a dimensional CAD drawing of the part. Electrical specifications describe the voltage and power a part is rated to handle so you know if it meets your needs. More complicated parts like chips also have detailed information about the function of each pin, and how the chip works.
The OOMP part page, part number, and back label give the most common characteristics of each part so you don't need to dig through the datasheet.
Bonus tip: when someone writes RTFM in a forum post it means Read The Fine Manual. Most questions answers are answered in the datasheet.