Allophone
A phoneme (such as /a/) is an abstract entity under which several different phones (actual speech sounds, such as [ä], [ɐ], [ɑ], [ʌ]) are classified in a speaker's mind. These differing phones are called allophones of one another (or of the phoneme).
There are two subtypes of allophones:
Continuous (phonetic) allophones
The human articulatory organs (and likely those of any physical being) are not perfect in operation. Since features such as the exact position of the articulators, intensity of phonation, the (speaker-dependant) shape of the oral cavity, and amount of food stuck between the teeth are continuous variables, any two attempts to articulate a sound will be ever so slightly different. Acoustics of the environment have further effect on the exact sound reaching the ears of an addressee. Yet, the brain ignores all this variation and recognizes a specific sound (this is proven to involve the recognition of specific formants' position and strength in the sound spectrum).
What exactly constitutes a "specific sound" is to some extent language-dependant. Two sounds that will be identical to one person's ear (say, [ɜ] and [ə]) will be typically recognized as distinct by someone speaking a language where the sounds are contrastive. However, not even this is a hard-and-fast division: depending on the degree of contrastive load, hearing or pronouncing a phoneme beyond its usual degree of variation may not matter, if context can be relied on to decipher the word.
Infants, it seems, will recognize a wide range of sounds, but will lern during language acquisition what sub-range of sounds to group under a given phoneme. The total variety seen in natural languages serves as a (loose) minimum indicator of what human hearing and articulation can possibly reliably distinguish.
There are no special IPA symbols for marking most continuous allophones, and the only exact way to record them is a recording of the sound itself; however, for a first approximation, diacritics such as raised/fronted/lowered/backed can be used.
