A piano-conductor score contains the *most important* (not necessarily all, since at times this would hardly be manageable – just imagine, say, Charles Ives’ orchestra works many of which require two orchestras to play in different tempi at the same time) orchestral parts arranged for piano (so-called piano reduction) along with the vocals (if any). The purpose is mostly to enable a pianist to accompany a vocalist during his/her preparation of a piece (opera, musical, etc.) which will later be rehearsed and performed with a full orchestra or at least those instruments prescribed by the full score (so-called correpetition).

A definition of piano score can be found at the Wikipedia reference below. A full score shows each part exactly as written for each player (it shows each part in the key that the player would use for his instrument. Scores are sometimes available with all parts transcribed as if every instrument were in the key of C). The piano score is the score reduced (simplified) to two or four hands and all parts transcribed to the correct key for the piano, which is in C.
The piano-conductor score is marked to show what instrument would be playing the part in the full version. See examples provided.

References:
A piano score (or piano reduction) is a more or less literal transcription for piano of a piece intended for many performing parts, especially orchestral works; this can include purely instrumental sections within large vocal works (see vocal score immediately below). Such arrangements are made for either piano solo (two hands) or piano duet (one or two pianos, four hands). Extra small staves are sometimes added at certain points in piano scores for two hands in order to make the presentation more nearly complete, though it is usually impractical or impossible to include them while playing. As with vocal score (immediately below), it takes considerable skill to reduce an orchestral score to such smaller forces because the reduction needs to be not only playable on the keyboard but also thorough enough in its presentation of the intended harmonies, textures, figurations, etc. Sometimes markings are included to show which instruments are playing at given points. While piano scores are usually not meant for performance outside of study and pleasure (Liszt's concert transcriptions of Beethoven's symphonies being a notable exception), ballets get the most practical benefit most from piano scores because with one or two pianists they allow unlimited rehearsal before the orchestra is absolutely needed. They can be used also to train beginning conductors. Piano scores of operas do not include separate staves for the vocal parts, but they may add the sung text and stage directions above the music.
http://en.wikipedia.org/wiki/Musical_score

#4 Sample Piano-Conductor Score.
http://www.musiccopying.com/PDFmusic/PCScore.pdf

Gaetano Donizetti - Lucia Di Lammermoor - Conductor's Score
Category: Any Instrument Sheet Music
Type: Conductor Score [see copy of a page of the conductor’s score]
http://www.encoremusic.com/oboe/1630315_alt_ge.html

• Fullness of Score
1. Vocal music:
a. With orchestral accompaniment
i. Full score and parts, if any, rather than conductor's score and parts, if any. …
ii. Conductor's score and parts, if any, rather than condensed score and parts, if any. …
b. Unaccompanied: Open score (each part on separate staff) rather than closed score (all parts condensed to two staffs).
2. Instrumental music:
a. Full score and parts, if any, rather than conductor's score and parts, if any. …
b. Conductor's score and parts, if any, rather than condensed score and parts, if any. …
http://www.copyright.gov/register/pa-best-musical.html

Once you have recognized and edited your music, you can extract and recombine parts and voices, create a conductor's score from separate parts, resize staves and systems or reformat the entire score. This chapter covers various ways of altering the ENF score structure….
Score Structure (Edit > Score Structure) is a powerful tool that allows you to create entirely new scores by removing, creating and re-linking parts. By simply dragging links around, you have the ability to remove parts, extract parts and join individual part scores into multi-part "conductor" scores.
http://www.musitek.com/SS5Manual/Output/Changing Score Struc...


--------------------------------------------------
Note added at 9 hrs (2008-01-09 16:54:14 GMT)
--------------------------------------------------

As shown in the examples below, the piano-conductor's score is not a full score and it contains information about the instrumentation.

Piano & Celeste (Piano-Conductor's Score sent with rehearsal material) Also available, at an additional charge: Full Orchestra Score (Partitur) in 3 volumes.
http://www.tams-witmark.com/musicals/brigadoon.html

The Vocal/Piano-conductor’s score for the Broadway standard…This Piano score contains full details of instrumentation, and also serves as a complete vocal score.
http://www.spartiti.biz/descrizione.asp?Ref=110311


--------------------------------------------------
Note added at 7 days (2008-01-17 05:37:42 GMT) Post-grading
--------------------------------------------------

Thank you, jomo :-) I'll keep you in mind.