The difference between a scientific observation and a scientific discovery is very simple: a scientific observation made for the first time is called a scientific discovery.
Scientific observations are made either to observe the workings of Nature or to test a hypothesis devised by a scientist. For example, consider the following scenario. A biologist spends some time in the wild to observe the behavior of lions. He does not have any particular idea about the hunting habits of lions. While in the wild, he observes that female lions tend to hunt in groups while male lions tend to hunt alone. Our biologist has made a scientific observation. If, supposedly, the biologist had in his mind the hypothesis (i.e. conjecture) that ‘only male lions do the hunting’ before he went to make his observations in the wild, then we will say that the scientist has conducted an experiment to test his hypothesis that ‘only male lions do the hunting’ and the experiment has refuted the hypothesis. In any case, the observation that female lions hunt in groups while male lions hunt alone would be termed a scientific observation. It would be a discovery for our particular scientist because he did not know this fact before, but it would not be termed a scientific discovery because it is something that is already known to the scientific community. However, the scientist who had made this observation for the first time and communicated his findings to the scientific community at large had made a scientific discovery.
While it is not necessary for scientific discoveries to be a bolt from the blue, they are often serendipitous. Some of the most ubiquitous and important drugs and chemicals were discovered by pure chance. Notable examples include plastic, Teflon (the nonstick substance that coats frying pans), penicillin (a potent antibiotic), Viagra (a drug used to treat male impotence), and some artificial sweeteners.
Scientists routinely consolidate scientific observations to form scientific theories. A scientific theory is a set of coherent principles that attempt to explain physical observations. Often, competing theories are put forward to explain the same set of observations. In due course of time, the theory that explains the observations more robustly will be favored over the others. Such a universally endorsed theory (e.g. Einstein’s Theory of Relativity) may also be termed a scientific discovery since the scientist who proposed the theory has actually discovered some part of the intricate workings of the physical universe. However, theories that explain the observed facts of Nature very well and find universal appeal among scientists are never the result of serendipity. On the contrary, they are the result of intense and prolonged mental concentration and deliberation.
It is interesting to note that some of the most mundane scientific observations have led to the discovery of some of the most important scientific theories of all time. Isaac Newton observed an apple falling from a tree and brought forward his Theory of Gravitation, which, together with his three Laws of Motion, form the basis of classical mechanics. Albert Einstein observed that the speed of light is constant regardless of whether the light is coming from a moving source (e.g. the headlights of a moving train) or from a stationary source. This observation led him to propose his famous Theory of Relativity. True scientific genius lies not in making accidental discoveries, but in formulating extraordinary theories out of ordinary scientific observations.