injures you. Unfortunately, the heavier types of armour also hamper your
melee speed, making it easier for monsters to hit you (i.e. reducing the
evasion score) and making it harder for you to hit monsters. These 
effects can be mitigated by a high Armour skill. Wearing heavy armour 
also increases your chances of miscasting spells, an effect which is not
reduced by your Armour skill.

injures you. Body armour (robes, mails etc.) come in two types: light
armour (e.g. robes and leather armours, also all elven armours) and
heavy armours (scale mails, ring mails etc.). Both help with damage
reduction but heavy armours do so more reliable.  Unfortunately, the 
heavier types of armour also hamper your melee speed, making it easier 
for monsters to hit you (i.e. reducing the evasion score) and making 
it harder for you to hit monsters. These effects can be mitigated by a
high Armour skill. Wearing heavy armour also increases your chances of 
miscasting spells, an effect which is not reduced by your Armour skill.

Body armours come in two types: heavy and light. Heavy armours give
some reliable protection from damage (in contrast to light armours) 
but have several disadvantages. 

you look at an armour's description with the 'v' command, this will in 
particular show you how cumbersome it is. This is measured by the 
Evasion modifier. Usually, all armours with EV penalty of 0 or -1 are
considered light. All others are considered heavy except for Elven 
armour (which is light). Walking and fighting in heavy armour will train
the Armour skill. If the Armour skill is high enough to offset the
EV penalty, even a heavy armour will train Dodging. Also, Armour skill
helps mitigate the bad effects of heavy armour on spellcasting.

you look at an armour's description (from within the inventory), you can
see in particular show you how cumbersome it is. This is measured by the
Evasion modifier. All armours with EV penalty of 0 or -1 are considered 
light, and all others are considered heavy -- except for Elven armour 
(which is light). 
Walking and fighting in heavy armour will train the Armour skill. If the
Armour skill is high enough to offset the EV penalty, even a heavy 
armour will train Dodging. Also, Armour skill helps mitigate the bad 
effects of heavy armour on spellcasting. A really high Armour skill will 
also increase the AC provided by other types of armour (gloves, cloaks
etc.).