Goblins are intelligent humanoid creatures that live in dark fortresses in the mountains. They are primarily interested in killing dwarves. They will siege any sufficiently populous or wealthy fortress, and frequently employ trolls in their armies to destroy doors and other buildings.
Occasionally, one or more goblin "snatchers" may arrive if you have children in your fortress. They are cowardly thieves, stealthy like kobolds, but their target is not your valued items, but the children of the fortress. Detecting them is announced by "Snatcher! Protect the children!". They will attempt to kidnap any children they come across and carry a bag with them for this purpose. After a few attempts, goblin master thieves may start showing up. Besides being generally more competent, these can also evade traps. There is also a small chance of getting the goblin ruler with these thieves, if he is a thief himself. Toady stated in a RPPR interview that these children will be raised by the goblins and can eventually show up in future goblin armies. Dwarven, Elven and Human converts are confirmed, though whether or not a child can return and siege his own fortress remains undetected.
Independent of this goblins will ambush your fortress, sending squads of different soldier types. Later, the sieges begin: your primary adversary in an invasion, goblin numbers increase every year once they start attacking, and eventually begin besieging your fortress more than once a year. Fortunately, they are not very bright, and will walk into traps by the hundreds. They may sometimes come riding beak dogs as cavalry. It is rare for goblin sieges to last until all of them die - the last few uncaged, un-eviscerated goblins will generally run for their lives rather than continue their attack.
Goblins carry somewhat valuable gear. They often wear giant cave spider silk clothing, iron armor, even steel armor (that can be melted down) and steel weapons. Crossbowmen will carry dwarf-usable bolts. Goblin clothing is considered "narrow" and dwarves cannot wear it, though it can be used as trade goods or melted. Goblin bones and skulls are not particularly valuable, although - as with all bones and skulls - the bones can be crafted into bolts and the skulls crafted into totems.
Unlike other races, goblin "roads" are underground, and if you get in to one, be sure you are ready — it can be a long walk to the other side, and you cannot enter the travel map when walking one. Being underground, they do not show up on the mapVerify.
 Living among them
Starting a fortress with a goblin stronghold in the local area is not for the faint of heart. In the current version of the game, the goblins will begin as friends to you. Be aware though, this is but a temporary ceasefire.
Because the goblin master thieves can avoid your traps, it's useful to chain a few war dogs at the entrance. Cage traps also seem to be more effective than other traps at catching those sneaking goblin masters.
As with Kobolds, it is possible to start next to a friendly goblin tower. This will add a great amount of wealth to your fortress, as well as provide you with potentially powerful allies.
In v40d, however, as soon as an invading force sieges you they will ALL turn hostile towards you, and will run out of their towers to slaughter you entirely.
 Starting Equipment
If starting in a goblin fort you should definitely ditch your anvil. If you have a soil level you can probably get by bringing less food and less booze, thus allowing you to bring lots of dogs; a large enough pack of dogs should allow you to escape the goblin fort with few casualties, and start building your own army. The dogs will also provide you with a nice source of meat.
 Goblin society
Goblin society has few laws. Slavery, assault, oath-breaking, and murder are all considered a personal matter between the parties involved. Additionally goblins view all forms of torture as acceptable (and fun).
Goblins, like elves, are immortal.
Goblins do not possess the aptitude for growing, tending more towards fishing, hunting, and butchering livestock for their food sources. Fortunately for goblins their digestive systems can derive as much sustenance from bones as from the meat that came on it.
[CREATURE:GOBLIN] [NAME:goblin:goblins:goblin] [TILE:'g'][COLOR:7:0:0] [GLOWTILE:'"'][GLOWCOLOR:4:0:1] [GENPOWER:2] [EVIL] [INTELLIGENT][LIKES_FIGHTING] [BONECARN] [CANOPENDOORS] [PREFSTRING:terrifying features] [BODY:HUMANOID:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH] [ATTACK:MAIN:BYTYPE:GRASP:punch:punches:1:2:BLUDGEON][ATTACKFLAG_WITH] [ATTACK:SECOND:BYTYPE:MOUTH:bite:bites:1:2:GORE][ATTACKFLAG_CANLATCH] [CHILD:12][BABY:1][MULTIPLE_LITTER_RARE] [SIZE:6] [FAT:2] [EQUIPS] [NOCTURNAL] [STANDARD_FLESH] [HOMEOTHERM:10067] [LAYERING:10] [SWIMS_LEARNED][SWIM_SPEED:2500] [PERSONALITY:ANGER:25:75:100] [PERSONALITY:IMMODERATION:50:75:100] [PERSONALITY:EXCITEMENT_SEEKING:0:60:100] [PERSONALITY:CHEERFULNESS:0:40:90] [PERSONALITY:ALTRUISM:0:25:50] [PERSONALITY:MODESTY:0:40:90] [PERSONALITY:SYMPATHY:0:25:50]