The term 'electromagnetic waves' denotes that these waves are composed of an electric and a magnetic part. Without going into too much detail, these parts are fields which exchange energy at a rate given by the frequency. The important bit here is that both fields exhibit an orientation in space. When the wave propagates in some 'forward' direction then the electric field can be orientated either vertically or horizontally, and the magnetic field is aligned perpendicularly to the electric field. The orientation of the electric field has been taken to define the polarisation of the whole wave. All this might sound like academic gibberish, but there are some consequences that are encountered in everyday life:

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  It is not easy to take photographs of people wearing glasses or sitting behind a window because the glass reflects light coming from behind the photographer. Experienced photographers know that sunlight consists of a colourful (in the real sense of the word) mixture of wavelengths, and that all colours are present in both horizontal and vertical polarisation. However, light that has been reflected from a glass window exhibits a pronounced polarisation. Thus, by mounting a polarisation filter to the camera and rotating it into proper position, this reflected light can be blocked and the object hidden behind the glass is revealed.
  

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  By playing with two polarisation filters, you can show that no light can get through when they are orientated at right angles to each other.
  

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  TV satellites can broadcast two stations on the same channel (read: frequency) at the same time, by transmitting one of them in horizontal and the other in vertical polarisation. A satellite receiver's antenna needs to be correctly orientated to be able to separate polarisations, otherwise these stations will interfere with each other.
  

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  Everybody setting up a radio receiver in their kitchen needs a couple of minutes to play around with its telescopic antenna until the position for optimum reception has been found. When switching from a telescopic to a loop antenna (eg. for 'short wave' radio), you'll find out that the loop antenna needs to be orientated at 90° with respect to the telescopic aerial and the line of sight to the broadcast station. This is because rod antennas detect the electric field, whereas loops detect the magnetic field.