The grey spheres represent carbon atoms, the red ones oxygen and the white ones hydrogen. The two blue spheres are nitrogen atoms and the single yellow sphere is a sulphur atom.
In the middle of the molecule is a square section, which gives penicillin its antibiotic effect. This is called a beta
This form of penicillin - also called Penicillin G - has a
at one end of the molecule. This contains a 6-membered benzene ring.