Trail running is all about freedom, but carrying some water, food or an extra layer will help you enjoy it more and let you push your boundaries on longer more technical runs.
An active pack should be comfortable to wear, have a snug fit so as not to bounce around too much and should be able to hold all the kit you need to take with you along the way.
In warm weather, staying hydrated is the key to maintaining your performance. A hydration pack is going to let you replace electrolytes, salts and water without the need to stop.
Hips packs are a great alternative to running backpacks when you only need to carry the essentials and not much else.