Attack/Defense Information
 

Attack/Defense(Att/Def or A/D for short) represents how likely you are to hit or be hit in combat.  If you have a very high A/D, then many smaller monsters will simply be unable to hit you.  The situation is the same for you, however, so if you are trying to attack a monster with a very high defense, then you may not be able to hit them unless your attack is comparably high.

A/D is divided up into Stats A/D, Item A/D and Guild A/D.
Item A/D will increase as you obtain and use more powerful items, better quality weapons having higher A/D, and better armour will give better A/D ratings also.

Stats A/D is dependent on your basic character statistics.(Makes sense)
A 1 point increase in Str & Dex will result in a 2/2 gain in A/D, and a 1 point WIS & INT increase will result in a 1/1 increase in A/D. Note that this includes modified stats due to items that increase/decrease your statistics.
E.g. Bronze plate has an A/D rating of 6/15, but has a -2 Dex adjustment.  This means that when you equip it, you A/D only increases by 2/11!

Guild A/D comes from your level in the guilds of which you are a member.  Your guild A/D will always come from the guild which has the highest A/D at any one time.    I do not know whether Att and Def are looked at separately when the game calculates which A/D is the highest (i.e. Att may come from a different guild to Def) or whether the totals of Att and Def are calculated and the figures which have the highest total are used.

It appears that all guilds except the spellcasting guilds surpass the A/D given by Artisan, but they take longer to get there.
More information regarding A/D at specific levels would be greatly appreciated.

At level 300:
 
 
Guild
A/D
Artisan
175/175
Warrior
200/218
Paladin
177/204
Villain
192/215
Ninja
196/196
Explorer
166/201
Thief
141/171
Barbarian
141/170
Healer
94/150
Sorceror
76/147
Magi
77/144
Warlock
68/145
 
Here is a more detailed table on A/D progression for the Artisan & Warrior Guilds.
 
Level
Artisan
Warrior
1
10/10
10/10
50
73/73
49/53
100
109/109
89/96
150
148/148
122/132
200
175/175
151/164
300
175/175
200/218
400
175/175
240/261
449
175/175
260/283

I have some information on other maximum A/D values, though these may be slightly incorrect.
 
Level
Guild
A/D
420
Paladin
221/255
457
Ninja
255/255
428
Explorer
208/252
395
Thief
158/204
515
Barbarian
179/203