What makes a parallel jaw clamp different from a trigger clamp?

The jaws stay parallel as they close, so they press evenly across a panel instead of angling in. That keeps glue-ups flat and square. Trigger clamps are faster but less precise.

Is the 39.8-inch bar long enough for cabinet work?

Yes for most cabinet sides, face frames and drawer boxes. For very wide glue-ups you may want a longer bar or a pair of these clamps.

Does it come as one clamp or two?

It is sold as a set of two clamps, which suits two-sided glue-ups.

Can one person tighten it easily?

The screw handle turns smoothly, but the clamp is heavy, so positioning it before tightening usually takes two hands.

Is it good for beginners?

It is a solid clamp, but its weight and screw adjustment suit woodworkers who do regular cabinet and furniture assembly more than occasional light use.