Since several people have asked, and since you won't know before you get to level 20, here's a "guide" to Armour Class. To see this at any time, type 'help AC'.
 

AC

Description

+101 and up

the rags of a begger (worst AC)

+100 to +80

improper for adventure

+79 to +55

shabby and threadbare

+54 to +40

of poor quality

+39 to +20

scant protection

+19 to +10

that of a knave

+9 to 0

moderately crafted

-1 to -10

well crafted

-11 to -20

the envy of squires

-21 to -40

excellently crafted

-41 to -60

the envy of knights

-61 to -80

the envy of barons

-81 to -100

the envy of dukes

-101 to -200

the envy of emperors

-201 and lower

that of an avatar (best AC)

*Information as of July 28, 2005