What are the various Buddhist scriptural designations and to what do they refer? In my readings so far (mostly Theravada based), I come across many scriptural references - for example, "AN," "MN 118," etc. To what do these designations refer? Is there some sort of concordance that summarizes the actual texts that these designations refer to?
They refer to various sutra collections within the Tipitaka.

Like for example, 


AN = Anguttara Nikaya
MN = Maddhiyama Nikaya


MN 118 would be the 118th sutta in the MN - which can be found in the MN index linked previously; or, Google is many times pretty helpful if you put in "MN 118 tipitaka" it should lead you to  http://www.accesstoinsight.org/tipitaka/mn/mn.118.than.html


  The Tipitaka (Pali ti, "three," + pitaka, "baskets"), or Pali canon,
  is the collection of primary Pali language texts which form the
  doctrinal foundation of Theravada Buddhism. The Tipitaka and the
  paracanonical Pali texts (commentaries, chronicles, etc.) together
  constitute the complete body of classical Theravada texts.    
  
  The Pali canon is a vast body of literature: in English translation
  the texts add up to thousands of printed pages. Most (but not all) of
  the Canon has already been published in English over the years.
  Although only a small fraction of these texts are available on this
  website, this collection can be a good place to start.    
  
  The three divisions of the Tipitaka are:    
  
  Vinaya Pitaka
  
  The collection of texts concerning the rules of conduct
  governing the daily affairs within the Sangha — the community of
  bhikkhus (ordained monks) and bhikkhunis (ordained nuns). Far more
  than merely a list of rules, the Vinaya Pitaka also includes the
  stories behind the origin of each rule, providing a detailed account
  of the Buddha's solution to the question of how to maintain communal
  harmony within a large and diverse spiritual community.        
  
  Sutta Pitaka
  
  The collection of suttas, or discourses, attributed to the Buddha and
  a few of his closest disciples, containing all the central teachings
  of Theravada Buddhism. (More than one thousand sutta translations are
  available on this website.)
  The suttas are divided among five nikayas
  (collections):
  Digha Nikaya — the "long collection"
  Majjhima Nikaya —
  the "middle-length collection"
  Samyutta Nikaya — the "grouped
  collection"
  Anguttara Nikaya — the "further-factored collection"
  Khuddaka Nikaya — the "collection of little texts":
  Khuddakapatha
  Dhammapada
  Udana
  Itivuttaka
  Sutta Nipata
  Vimanavatthu
  Petavatthu
  Theragatha
  Therigatha
  Jataka
  Niddesa
  Patisambhidamagga
  Apadana
  Buddhavamsa
  Cariyapitaka
  Nettippakarana (included only in the Burmese
  edition of the Tipitaka)
  Petakopadesa (  "   "  )
  Milindapañha (  "  "  )        
  
  Abhidhamma Pitaka
  
  The collection of texts in which the underlying doctrinal principles
  presented in the Sutta Pitaka are reworked and reorganized into a
  systematic framework that can be applied to an investigation into the
  nature of mind and matter.


http://www.accesstoinsight.org/tipitaka/