Circ needles or DPNs?

Hi. I am almost finished with my first sweater. I have two questions I would love some help with.

I finished the front, back, and two sleeves. All were done on straight needles. Now it says to “sew raglan seams” What is the exact definition of raglan? What parts am I sewing together? Because next I am picking up the remaining sts to start knitting a collar and once the collar is finished then it tells me to sew sleeve and side seams. This sweater is an off the shoulder sweater with a collar that folds down at the shoulders.

My next question is that it tells me to pick up all the sts on circular needles. Can I use DPNs instead or should I buy circ needles? Will it matter?
Thank you all so much!!!

You won’t be able to fit all the stitches on dpns for the collar.

The raglan seams are the slanted ones from the neck to the underarm.