Wines are rated by different publications, each with similar, although slightly different scales and definitions. Here are a few from the major wine rating publishers:

Wine Spectator
A 50 Point Scale and what it indicates:

95-100 - Classic; a great wine
90-94 - Outstanding; superior character and style
80-89 - Good to very good; wine with special qualities
70-79 - Average; drinkable wine that may have minor flaws
60-69 - Below average; drinkable but not recommended
50-59 - Poor; undrinkable, not recommended

Wine Advocate
Also a 50 Point Scale and what it indicates:

96-100 - Extraordinary; a classic wine of its variety
90-95 - Outstanding; exceptional complexity and character
80-89 - Barely above average to very good; wine with various degrees of flavor
70-79 - Average; little distinction beyond being soundly made
60-69 - Below average; drinkable, but containing noticeable deficiencies
50-59 - Poor; unacceptable, not recommended

Wine Enthusiast
A 20 Point Scale and what it indicates:

95-100 - Superb. One of the greats.
90-94 - Excellent. Extremely well made and highly recommended.
85-89 - Very good. May offer outstanding value if the price is right.
80-84 - Good. Solid wine, suitable for everyday consumption.

While ratings may influence your purchasing decisions, what really matters in the end is your personal feeling about the wine.