tl;dr for interplanetary launch windows: due to the nature of orbits, spacecraft can only launch for other planets at certain times.
To reach objects in Earth orbit, the launch window really doesn't matter much. We orbit around the Earth so quickly, if we miss the transfer window for, say, the moon during one orbit, we can just wait 90 minutes or so for the next launch window.
The International Space Station orbits the Earth once every 92.69 minutes. Orbits of Kerbin are even faster.
But when going to another planet, the orbital period that matters is not the time it takes our ship to orbit the Earth. The orbital period that matters is the time it takes the Earth to go around the Sun. And we only orbit the Sun once a year. If we miss a transfer window, we have to wait many days for the next.
The next transfer window might be more than a year away or less than a year away, depending on how fast the other body is orbiting the Sun. But no matter which other planet you are going to, it's going to take awhile.
Also the relative orbital inclinations of planets makes some transfers impractical.
(If we had warp engines from Star Trek, or any other super-fast sci-fi engine, the transfer window wouldn't really matter much. We would have so much deltaV we would just brute force our way wherever.)
So anyway, the POINT of all this is that we had a transfer window to Duna (Mars) while New Dawn was still on its way to Eve (Venus).
So we designed our second interplanetary ship Ambition for a simultaneous Duna mission. Here's Ambition:
Ambition weighs in at just under 33 tons. It is composed of three autonomous spacecraft.
From top to bottom:
Maven: This probe is designed to land on the surface of Duna's moon Ike. It will probably stay there indefinitely.
Ambition Lander: This will be the first manned Kerbal spacecraft to land on another world! (Hopefully it will also be the first manned Kerbal spacecraft to return from the surface of another world.)
Ambition Tug: This is a tug. A tug's sole purpose in life is to move another spacecraft from point A to point B. Ambition Tug is designed to take the Ambition Lander to Duna and back.
The rocket that will get Ambition to space is the unimaginatively named KR-2L Booster. The first stage is 12 solid rocket boosters (SRBs), followed by the second stage: a KR-2L engine with 4 big orange tanks full of fuel. This second stage is a fully functional spacecraft in its own right. It will make it to orbit then separate from Ambition and land on its own.
The KR-2L Booster can get ~35 tons to Low Kerbin Orbit (aka LKO aka Low Earth Orbit aka LEO).
In general, this basic design of SRBs on the bottom with a fully recoverable second stage has served me well in career mode. The SRBs aren't be recovered, but they are cheap, so that's not a big loss.
The second stage is smaller and more efficient than it would be without the SRBs. It handles pretty well and lands easily. Without SRBs, the second stage would grow so huge that it would become clunky, difficult to control, and difficult to land.
The Ambition launched without incident. The second stage separated without incident and landed safely. Then the Ambition burned for Duna intercept with its three nuke engines.
Half a year later or so, Ambition entered Duna's gravity well.
A small course adjustment was made so Ambition would flyby Ike.
We make one more small course adjustment so Ambition would use Duna's atmosphere to aerobrake and remain in orbit around Duna, instead of flying back toward Kerbin.
ACK! ERROR! Ambition did not go nearly low enough into the thin atmosphere of Duna. The aerobrake didn't slow Ambition down enough for orbital insertion.
Engines to the rescue! Ambition fired its three nuke engines. This wasted a fair amount of fuel, but there should still be enough in reserve to complete the mission.
Ambition became the first Kerbal spacecraft to achieve orbit around Duna. Party!
We did one more aerobrake to drop Ambition into a lower orbit of Duna.
The lower orbit was good for reaching Duna's moon, Ike. Maven probe detached!
Now time for the Ambition Lander to land on Duna proper:
Success! A Kerbal has set foot on another world for the first time! Party!