There are different types of editions for printed music and which you choose depends upon what you need it for. You might need more than one edition of a work for comparison purposes.

• Critical edition: often referred to collected works or monuments, these editions include scholarly background information and commentary about the works and are carefully edited. Usually found in the M2s and M3s in the Reference section, these items don't circulate and are not meant to be performed from in most circumstances.
• Performing edition: these are the scores that you will find on the second floor of MPAL in our circulating collection. These vary greatly in quality based on publisher and editor, and your studio professor will likely have strong opinions about which ones you should use. Some include a lot of editorial marks (bowings, breath marks, fingerings, etc.) and may be unique to that editor (often a famous performer). Editions from publishers like Henle and Wiener Urtext are generally of good quality, while editions from publishers like International Music should be used cautiously. 
• Urtext: "A term used in studying and editing musical sources to signify the earliest version of the text of any composition, musical or otherwise, a version that is usually no longer extant; it is also used to signify a modern edition of earlier music which purports to present the original text, without editorial addition or emendation." (from Grove Music Online)

The Library now has access to Music Online: Classical Scores Library. These online scores can also be found through the library catalog. 

The Details section in a catalog record provides important information about the format of the printed music, specifically the Genre and Format sections. 

Full score

shows all instruments/voices in a work of music.

Miniature score (or study or pocket score)

Shows all instruments/voices in a work of music, but produced in a small size. Call numbers start with "MM" and are shelved separately from the other scores in our collection.

Vocal score (or piano-vocal score)

for operas or other choral/orchestral works; provides the vocal parts and presents the instrumental parts in a piano reduction or accompaniment.

Score and parts (or just parts)

For chamber music (2 or more players/singers) the printed music may be a score, showing all parts together, and/or include separate parts, one for each performer and their voice/instrument.

Just the parts and no score:

Or, a quartet with score and parts included:

Piano reduction or arrangement

Usually for concertos, presents the soloist's part and and presents the orchestral parts in a piano reduction or accompaniment.

Arrangement

when a work is originally written for one instrument or instruments and is arranged for another instrument or instruments.