There is no any exact definition for Bandhas & Mudras. But depeneding on their Nature Of Practice we can define as Asanas,Bandhas,&Mudras are particular postures to be retained; and Pranayamas, ShatKriyas are Actions to be performed . This is the main difference between these Yoga Kriyas. Bandhas are particular postures involving pressure on muscles,which indirectly implies same pressure to that particular corresponding regional nervous system.But, one has to remember,Siddhi in practice of Pranayama is attained through the help of Bandhas and Mudras only.
Bandhas and Mudras does not have clear distinctive defference between each other in practical scenario.
Bandhas and Mudras may or may not be able to give complete advantages, when they paracticed individually.Due to of this aspect it is adviced allways ( in all the systems of Yoga) to practice in combination with any particular Pranayama Process and along with a Aasana.
This means, when a sadhak want to get a particular or specific advantage, one has to combine corresponding Asana with Bandha and a Mudra with one specific technique of Pranayama.This one entire Combination will definately manifest complete advantages. Asanas and Bandhas Or Mudras can give great results "individually",but, the effect of greatness will be enhanced, and will become specific when a combination is maintained. When Physical fitness is not cooperating, to retain the effect, sadhak can practice any one either asana or bandha or mudra for a specific period,because practice of any one action in the combination will also potentiate or rejuviante total Effect.This is the greatness involved in HathaYoga.
Here we are furnishing the detailes of Mudras & Bandhas together, but, not as two separate categories,because, as how I said above, asanas,bandhas & mudras has to be practiced in combination for effective results. And other important reason for combined presentation is, in some instances one bandha or mudra may be the proceeding posture for the next one, i.e.,if,once one bandha or mudra is obtained, from that posture to get another posture is easier, than, practicing the other one individually. For Example,practicing Maha Mudra, Maha Bandha, Maha Vedha together is easy than practicing separately or individually. Because Maha Mudra posture can be converted into Maha Bandha with a small change in posture, like wise,Maha Bandha can be converted into Maha Vedha with a small change inposture.
Even though there are many Mudras and Bandhas are explaind in HathaYoga Pradeepika and Gheranda Samhitha, due to their nature of practice and due to of other reasons,,many of them last their entity.Here you can find detailed discription of most widely accepted and practiced Mudras and Bandhas.