Precision increases your chance to critical hit and like Power, is an offensive attribute. At level 80, all professions have a chance to critical hit of 4%. This is incredibly low but it is easily increased through items or trait lines. The main benefit of Precision, besides the fact that critical hits are great for spike damage, is that many effects from traits or skills are triggered by critical strikes.
Certain traits will directly increase your Precision attribute, such as a Mesmers Blade Training that provides +50 Precision, while skills such as an Engineers Infused Precision will only trigger when you critically hit.I always try to aim for around 50% chance to critically hit as many of guild wars 2 gold around such trait triggers. With the rate of attack/fire from many skills or weapons it’s almost guaranteed with such a Precision rate that you can keep trait triggers active. To achieve a 50% chance to critically hit, you would need a Precision attribute total of around 1800.
Toughness is one of two defensive attributes. Each point spent in Toughness will increase your armour and your ability to withstand direct damage. Toughness doesn’t affect your ability to mitigate Condition damage but only physical attacks from weapons or spells. Similarly to Precision, there are various traits for certain professions that provide a direct increase, such as a Warriors Shield Master trait which provides +90 Toughness while wielding a shield. There is no rule as to how much toughness, numerically, that you should aim for. With very little Toughness you can still survive a long period of time if you are clever with your positioning and use of dodging in order to cheap gw2 gold, but anything around 1600 Toughness at level 80 should see you withstand plenty of damage, when combined with your armour.