In phonetics, aspiration is the strong burst of air that accompanies the release of some stop consonants. To hear and feel the difference between the aspirated and the unaspirated sound, put your hand in front of your mouth and say key and then ski. The [k] in Key is aspirated; in ski it is unaspirated.
English voiceless stops are aspirated when they begin a stressed syllable (as in pen, ten, Ken), but this aspiration is not distinctive (they also have unaspirated variants in other positions). In many languages, for example Hindi/Urdu and Ancient Greek, [t] and aspirated [th] (not to be confused with the English sound spelt th as in thin) are different phonemes.
Basel German has unaspirated /p, t, k/ and aspirated /ph, th, kh/; the latter may be viewed as clusters. Icelandic has pre-aspirated /hp, ht, hk/; some scholars interpret them as clusters. /b, d, g/ are normally also voiceless in Danish and most Southern varieties of German. Traditionally, they are transcribed as /b, d, g/ by scholars (especially in Danish linguistics), even though what distinguishes them from their "fortis" counterparts /p, t, k/ is their lack of aspiration rather than their voicing (which is usually absent).