Vacuum collection tubes are glass or plastic tubes sealed with a partial vacuum inside by rubber stoppers. 
The air pressure inside the tube is negative, less than the normal environment. 
After inserting the longer needle into the vein, the phlebotomist pushes the tube into the holder so that the shorter needle pierces the stopper.
The difference in pressure between the inside of the tube and the vein causes blood to fill the tube. 
The tubes are available in various sizes for adult and pediatric phlebotomies
 
Different blood tests requires different types of blood specimens.
Most tubes have additives called anticoagulants which prevent clotting/coagulation of the blood.
Plastic tubes may have an additive to enhance clotting of the blood 
Anticoagulants are already in the tubes in the precise amount needed to mix with the amount of blood that will fill the tube.
The color of the stopper on each tube indicates what, if any, anticoagulant the tube contains. 
It is important to completely fill each tube so that the proportion of blood to chemical additive is correct, otherwise, the test results may not be accurate or the specimen will be rejected and will need to be recollected.
It is also important to thoroughly mix the blood with the additive by gentle inversion