Are all capacitors polarised?

No, many types are non polar, so they can be put into a circuit in either direction. The practical difference between polar and non-polar capacitors is that the polar types generally have higher capacitance values, while non-polar types can often withstand higher voltages. WARNING: If a capacitor breaks down due to high voltage or using wrong polarity, it can explode, sometimes violently. At the same time, the contents can be vapourised and cause smoke to come from the circuit. This smoke can be toxic. In such an event, turn off power immediately and avoid breathing the vapour. Then vacate the area until the smoke has dispersed.

Aluminium Electrolytic Capacitors

Ideal for use in filtering or smoothing applications in power supplies. Also used for coupling and bypassing in audio circuits and as a timing element in non critical circuits.
Modern aluminium electrolytic capacitors have high reliability and low leakage. Special low leakage versions are available with leakage which rivals that of tantalum capacitors.

Solid Tantalum Capacitors

Offer smaller size and lower leakage than standard.

Ceramic Capacitors

Offer low cost and high capacitance in a small physical volume.
There are generally two types:
1) High stability, temperature compensating types for use in resonant circuit and filter applications. These have linear temperature characteristics, and their value is largely independent of voltage and frequency.
2) Bypass and coupling capacitors for use in less critical applications. These are less stable, have nonlinear temperature characteristics and are somewhat voltage dependent.

Paper Capacitors

An original construction, now rarely used. Plastic film capacitors have replaced paper capacitors in most applications.

Polyester Capacitors

Low cost, good stability and available in a large range of values. These are the most widely used capacitors for general purpose applications.
Greencaps and MKT type capacitors are examples of polyester (polyethylene terephthalate) film capacitors.

Polycarbonate Capacitors

Offer low temperature coefficient and lower dielectric losses at high frequency. Most often chosen for temperature stability characteristics.

Polystyrene Capacitors

Usually chosen for applications requiring tight tolerance coupled with high stability. Predictable temperature coefficient used in conjunction with particular ferrite cores makes highly stable tuned circuits or oscillators.

Polypropylene Capacitors

Offer very low dielectric losses and good temperature coefficient. Used in power electronics applications, e.g. mains capacitors, switching power supplies, inverters and TV deflection circuits.

RF Mylar Capacitors

A modern type, suited to high frequency applications.

Variable Capacitors

These vary in form from the old multi vane rotating capacitors used in early radios to this modern specialised version, rated at 10,000V. Suited to tuning and fine adjustment applications.

Miniature Capacitors

A modern type, developed for high density circuit boards, particularly computer circuitry and digital receivers. These are usually assembled by automated plants in large numbers, often to custom designs.

Capacitor Markings

Capacitors may be marked to show their value, voltage rating, accuracy, temperature stability and other information. Most capacitors are not marked with all of these, however, the value and voltage rating are usually given. Identification can be difficult because of the variety of systems in use.

Film Capacitor Types

Units

The unit of capacitance is the Farad, but this unit is too large in practice. Commonly used smaller units are the microfarad (abbreviated µF), nanofarad (nF) and picofarad (pF). The section on decimal multipliers shows the relationship between these.

Decimal multipliers

Decimal multiplier prefixes are in common use to simplify and shorten the notations of quantities such as component values.
Capacitance, for example, is measured in Farads, but the Farad is far too large a unit to be of practical use in most cases. For convenience, we use sub-multiples to save a lot of figures. For example, instead of writing 0.000000000001 Farads, we write 1pF (1 picofarad).
The more common prefixes and the relationships to one another are as follows.

Value

Larger capacitors are marked in microfarads and indicate this by the abbreviations 'µF' or 'µ' or even the obsolete 'MFD'. Smaller capacitors are marked in nanofarads or picofarads and may abbreviate the unit to 'n' or 'p'.

If the value contains a decimal point, the 'µ', 'n' or 'p' is sometimes put in place of the decimal point. Therefore, a 4.7pF capacitor can be marked as 4p7.

If no unit is given, a judgement, based on the capacitor's physical size, must be made to determine which unit is intended. For example, a small ceramic capacitor marked 4.7 is probably 4.7pF, whereas a large plastic capacitor marked '4.7' is more likely to be 4.7µF. If the value is in nF then this is invariably shown.

Another marking system uses 3 numeric digits to indicate the value in picofarads. The first two digits represent the first two digits of the value and the third digit is the multiplier or the number of zeroes.

For example, a capacitor marked 104 would be read as 1, 0, 0000. This would be formatted as 100,000 pF and would commonly be known as 100nF or 0.1µF. Likewise, a capacitor marked 472 would be 4700pF, also known as 4.7nF or .0047µF. A similar system represents these 3 digits using colours taken from the resistor code, instead of numbers.

Some common values and their possible markings:

Voltage Rating

Polarity