It is best to do a short warm-up before strength training, and save the cardio for after. The warm up is an important strength training preparation for multiple reasons: faster contraction and relaxation of the muscles, improvements in rate of force and reaction time, increased blood flow to muscles, improved oxygen delivery, and improvements in muscle strength and power. If you are doing both strength and cardio back to back, you will lose fewer benefits in the cardio session for having already done your weights than you would lose in the strength session if you went into it fatigued from your cardio, and possibly difficult to maintain good form throughout. I recommend doing the cardio session after strength for advanced exercisers and on a different day for beginners.
There is no right or wrong answer.
It depends on your goals, and what your workout for the day is.
Yes, some light and/or easy cardio is recommended to “prime” the muscles for a workout session.
Besides for just giving my clients a good solid workout, I also try and educate them and give them the understanding in how to build a good workout.
From the warm-up, all the way through the cool-down.
To me, it definitely depends on training goals. I always recommend at least a 5-10 minute warm up of some type of aerobic activity before strength training for everyone – regardless of goals & training levels. I don’t see a problem with doing 20-25 minutes of cardio before moderate strength training – in fact, I encourage it. They might as well extend the warm up another 10-15 minutes and knock it out. However, if gaining strength & muscle mass is the primary goal, I would suggest doing the cardio workout after hitting the weights and/or during off days. That way, you won’t be too fatigued when training with heavier weights.