Marksmanship Hunter, much like Beast Mastery, might have their changes overshadowed by Survival's radical redesign. Marks is still a ranged spec that deals mostly physical damage, and by default, they still have a pet (though they are uniquely the spec with "Lone Wolf" as a talent.) So superficially, the spec looks similar to what we've had before.
However, in fact, the spec has received some pretty big changes. Let's go over them!
Focus for all Hunters has been changed to regenerate faster. I believe that at this point it's functionally identical to Energy, but it's still a brown/orange bar for Hunters.
Arcane Shot and Multi-Shot now generate Focus, rather than costing it. Arcane gives you 5 Focus while Multi gives you 2 per target hit. Neither has a cooldown, which of course makes these clear filler abilities any time you can't do your other stuff.
Set Ups to Set Ups:
So here's where things get interesting and arguably weird:
Your auto shots have a chance to make your next Arcane or Multi-Shot put Hunter's Mark on anything they hit.
Having a Hunter's Mark then activates Marked Shot, which costs 30 Focus and hits any targets with Hunter's Mark within range for some physical damage but then puts a debuff called Vulnerable on them.
Vulnerable lasts 30 seconds and increases the damage taken by Marked Shot and Aimed Shot by 25%, stacking up to 3 times (and refreshing when gaining a new stack.)
So that's a proc that tells you to hit one of two abilities (depending on the number of targets,) which activates another ability that buffs itself and another ability.
Are you following?
Your biggest-hitting ability, especially when there are three stacks of vulnerable on the target, is Aimed Shot, which costs 50 Focus and has a 2.5-second cast time (of course, faster with haste.) You can't cast Aimed Shot while moving, so you will have to occasionally stand still.
Your new major cooldown is Trueshot, which has a 3-minute cooldown and lasts 15 seconds. It increases your Haste by 40% and causes every Arcane Shot or Multi-Shot to apply Hunter's Mark.
Your new mastery increases the range on all of your ranged abilities and also increases the damage of your Focus-spenders (so Aimed and Marked Shots) by a fairly large amount.
Of course, you can add abilities to the rotation via talents, but the core to the gameplay is going to be applying Hunter's Mark and then using Marked Shot to keep up the Vulnerable debuff on your targets. It is a very reactive spec, but grants you a great deal more focus on the ranged physical attacks of the spec.