Note: This is a rewrite of a message I wrote to the Parallax Basic Stamp Mailing List, but it seemed generally interesting. This was all off the top of my head, so it may or may not be very complete.
There are many factors that measure the quality of a cap. Selecting a capacitor means maximizing the factors you need while still keeping the other factors reasonable. What are the factors? My list looks like this:
Leakage and other resistance (and impedance) effects leads to lots of other
measurements including Q, power factor, and Effective Series Resistance (ESR). These are
all useful ways to measure what resistance is doing to your capacitor. An
For many circuits this isn't a big deal because the values are inconsequential. But at microwave, or micropower levels they may be important.
So what are the practical ramifications:
Of course, there are exceptions. Some electrolytics have better designs than others. There are ceramics built to be stable over certain temp ranges (known as the temp coefficient). Others have leads designed to minimize inductance.
So, how to proceed? You need a capacitor of a given value. You can ask yourself:
1) How stable does it need to be?
2) How large can it be?
3) How much leakage can I tolerate?
4) How much temperature sensitivity can I stand?
5) How much will I pay?
Take a filter capacitor for a power supply. We don't care exactly what value the cap is. We just want a big one. That's why electrolytics are popular in this application. They tend to put the most capacity in the smallest container at the lowest cost. When I designed very small instruments for the DoD, we would often use tantalum which can be quite small, but costs much more.
Bypass capacitors are another place where the exact value isn't that important. You don't need big capacity values, so ceramic is very popular here.
If you are building a tuned circuit (say an LC tank) or an oscillator, you want a capacitor with tight tolerances and low temp sensitivity. Polystyrene is good here if you can operate without heat over 80C (which is actually pretty hot). Silver Mica is very good, if you can afford the space for a bulky capacitor. A carefully selected ceramic with good temperature coefficient at the temperature of interest will save money and space, but will probably never be quite as good as polystyrene, teflon, or silver mica.
So you can see this is a bit of an art. You have to get the best performance where it counts while still keeping the other factors reasonable. You can usually replace a "bad" capacitor with a "better" capacitor. For example, you could easily replace a ceramic capacitor with a poly film capacitor, all other things being equal. You can always use a capacitor with a higher working voltage than the one you are replacing too. However, you usually don't want to trade "down". Don't replace a silver mica with a garden-variety ceramic, for example. Watch out for polystyrene if you are in a high temp environment (automobiles, for example). Don't put polarized caps of any kind where a non-polar is called for. Going the other way should be OK, since I don't know of a rational designer who would take advantage of the polarizing effect (I could be wrong, though -- it would be possible).
Some other links: