A continuous spectrum: a graphic representation of the spectrum of visible light. (Deborah S Krolls, wikipedia)
Emission spectra: a graphic representation of (top to bottom) the emission line spectra of hydrogen, nitrogen, and iron. Note how simple the hydrogen spectrum appears, compared to the complex spectra of the more complicated atoms. (Images from wikipedia)
An example of how spectra (in this case, an emission line spectrum) are observed (NASA, ESA, and the Hubble SM4 ERO Team)
On the left side of the montage above is an image of Eta (h) Carinae, a binary star system in which the more massive star is ejecting mass at a tremendous rate (about one solar mass per millennium), and is within a million years of blowing itself to bits. A vertical line in that image shows the region where light from the gas ejected by the star has been passed through a spectroscope, to observe the emission line spectrum shown, with labels, on the right. The NASA news releases show the spectrum as a colored image; but it was actually a black and white image, and is therefore shown as such, below. (False) colors are often added to published spectra to make them more attractive; but the positions of the emission lines determine their wavelengths and apparent colors, and adding color to a spectrum is neither necessary nor desirable when studying it, because it makes it harder to discern faint details.
The (emission-line) spectrum of Eta (h) Carinae (NASA, ESA, and the Hubble SM4 ERO Team)
A digital map of the spectrum of the Sun, colored to resemble an actual spectrum.
The above image is a high resolution version of the solar spectrum. Each horizontal strip covers sixty Angstroms of wavelength, decreasing from 7000 Angstroms at top left to 4000 Angstroms at bottom right. Tens of thousands of absorption features, at specific wavelengths corresponding to absorption by gases of a particular ionic species, are scattered across the background of an essentially continuous, black-body spectrum. Analysis of the strength of different absorption lines, and of the shape of the lines tell us much about conditions in the solar atmosphere, and allow us to estimate relative abundances of the seventy elements whose absorption lines are observed in the Solar spectrum. The absorption lines of those elements are seen at the same wavelengths where emission lines would be seen, if the gases producing the lines were emitting light, instead of absorbing it. In reality, they do both. They absorb light coming toward us from the "surface" of the Sun, reducing the amount of light that reaches us, thereby producing dark "absorption" lines. But in the next instant, they reradiate the light, often (although not always) at exactly the same wavelength, but in a random direction, so that only a small portion of the absorbed light happens to be heading in our direction. If we were looking at the Sun from another direction, and saw the reradiated light against the background of space, we would see an emission spectrum. (Nigel Sharp (NOAO), FTS, NSO, KPNO, AURA, NSF, apod060423)
Another view of the solar spectrum, showing how Fraunhofer labeled the stronger absorption lines, from red to violet, with various letters and numbers. Stronger absorption lines received capital letters from A to K (skipping I and J, because they are so similar in Germanic script), slightly weaker lines received small letters from a to h. The numbers at the bottom of the image correspond to the wavelength of various kinds of light, measured in Angstroms (millionths of a centimeter). (Modified version of image at Spacetech's Orrery, original source not specified)
A comparison of the hydrogen emission spectrum with the Fraunhofer absorption lines in the solar spectrum. The hydrogen emission lines are called (from right to left, or red to violet) H-alpha, H-beta, H-gamma, and H-delta. The H-alpha and -beta emission lines correspond exactly to the C and F Fraunhofer lines in the solar spectrum, and the H-gamma and -delta emission lines correspond exactly to the f and h Fraunhofer lines, thereby demonstrating the presence of hydrogen atoms in the solar atmosphere. (Combination of Spacetech Orrery image above, with a hydrogen emission spectrum)
