A cat may strike a submissive pose because she is scared or nervous. This will generally include holding her tail low or between her legs and holding her head pointed downwards and possibly even prostrating herself entirely before you.
Between two cats who do not know each other, or who do not get on very well it is common for one to be submissive towards the other to prevent the beginning of hostilities. However, this can also be a ruse and the apparently submissive cat may still launch an attack.
A cat may also strike a submissive pose to show you how safe they feel. Rolling on to her back and exposing her tummy to be rubbed is showing you not only that she loves you rubbing her belly, but also that she trusts you sufficiently to be vulnerable in front of you. Similarly, exposing her neck to be stroked is a sign of trust and affection. However, be warned, some cats will expose their bellies, only to grab any unsuspecting hand which dares to come in for a stroke with all four paws.
When a cat feels aggressive and wants to look scary, he will arch his back and puff up his tail to make himself look bigger. It is very easy to recognise this pose, which is often accompanied by hissing and screeching. Do not approach a cat striking an aggressive pose such as this, even if you know them well. They are unlikely to react in a friendly manner.