Fixed wing pilots have to figure out a means of getting in the air. Because they rely on gliding past large volumes of air to fight gravity, unless they're moving at a certain minimum speed relative to the air they will stall, and fall to the ground. How much thrust they need to sustain level flight, or to climb, is a function of their aerodynamic performance, their weight, and the thrust generated by the motor (which in optimal conditions is closely related to motor power). Because this is relative to airspeed rather than ground speed, it is always easier to take off pointing into the wind.
Lacking an electric motor for cruise, all the energy a glider on flatland has is the energy imparted at launch, and any scant lift they can gather from thermals. Launch methods are essential for gliders, and these were all developed extensively before electric planes became popular, when model plane builders could either invest in the complexity, noise, and mess of a nitro engine or go without power.
 Electric Planes
There is a continuum of planes from those which barely have enough power to stay level, which rely heavily the lift generated by gliding at cruising speed, to those with motors and props powerful enough to take off vertically and essentially ignore aerodynamic lift. Acrobatic planes are defined as those on the more powerful side of that continuum, while sailplanes are on the less powerful side. Engineering-wise, the thrust to weight ratio and the degree of induced drag are the main figure of import, but absent good ways of measuring drag, most people use the angle of the maximum sustainable climb. A plane that can climb at 10 degrees with wide open throttle is very underpowered, while a plane that can climb at 80 or 90 (vertical) degrees will do nonsensical aerobatic maneuvers without breaking a sweat.
The tradeoff is in weight and structure. Powerful motors, and airframes which can tolerate high speeds, are very heavy, and shorten flight time significantly. For most UAV tasks, these aerobatic capabilities are not desirable because of this effect on flight time; The system must be as light as possible to stay in the air longer, and there is significant deadweight used by the battery, FPV system, or camera.
The main limitation on heavy, powerful planes is not takeoff, but landing. Light planes, even balsa ones, can land into the wind onto a grass surface and endure no damage at all very reliably. A heavy one may require a difficult tail-down stall or even a genuine runway in order to get rid of its velocity without damage. Intelligent use of flaps make this somewhat easier, as the fast aerobatic plane can suddenly become a much slower-moving one which generates a lot of lift at the end of its flight.
Given a rough landing area (or one in which you cannot constructively use a headwind), even absent considerations of flight times, a lightweight/underpowered model may the only thing practical. If this area isn't accompanied by a hill, or has a limited flat area, launching problems may ensue.
 Climb Rates
A vertical-climbing plane doesn't need to worry about takeoff at all because it can sustain its weight without any forward airspeed. A plane that can do significant but non-vertical climbs is going to accelerate up to a speed where it can sustain altitude very fast.
A non-aerobatic plane that can climb at 10 degrees with wide open throttle is going to lose significant altitude while it's moving at slower than cruising speed. This means that from the time it leaves the pilot until the time that it reaches cruise speed, it's dropping towards the ground.
Even a plane that is aerobatic may have problems launching in a shortened runway (for example, a clearing in the trees), where it needs a large climb rate to pass obstacles.
On a hill, planes like this are easy to launch because they can drop towards the ground slower than the ground recedes away from them.
In a flat area with a strong wind, the plane *starts* at cruise airspeed even at zero groundspeed.
Pull back, start running, release. Usually adequate on an electric plane with a motor powerful enough to do fun aerobatics on a flat, clear landing zone.
 Discus Launch
A knob at the end of one wing allows one to use the entire strengthened plane like a slingshot, accumulating lots of kinetic energy before leaving the hand.
 Cart Launch
Larger planes which don't have a landing gear sufficient for the terrain may launch from a rolling cart or dolly, pulled by a human, perhaps with a pulley. Landing gear on a small plane is a deadweight loss without a good runway, as it is nonfunctional in even rough grass, so building a launch-only carriage with big, heavy wheels, combined with a grass landing, is not an uncommon solution. Rarely, full-scale planes rely on carriages when fitting a landing gear in is not practical, like the Boing Phantom Eye.
 Elastic / Bungee Launches
Usually, a bungee is not strictly rubber; This would either provide too powerful of a pull for longer lengths, or provide too constricting of an angle (with too much ground-wards component rather than horizontal pull) for shorter lengths. Several times the length of rubber is usually used in less stretchy monofilament or twine.
A bungee launch with no incline and no camber is known as a 'zip start'.
The slip ring for these systems is a safety hazard, and may be equipped with its own parachute or ribbon to bleed off velocity. Full-rubber bungees are even more dangerous, as lots of energy is stored bringing the bungee up to the length where it has enough force to pull the plane.
 Bungee Catapult
A silicone-rubber band (usually the ubiquitous 'surgical tubing') is pulled back along a slightly inclined two-rail track, and then released. The catapult gives the plane *just* enough elevation for the bungee to continue to increase speed and lift to cruise speed. Planes with weak points at the rails should take caution. A standard mechanism for this type of launch has been developed by the RC community.
 High Start Launch
A very long bungee, and a very long length of inert cord can generate a long-lived horizontal pull. This pull can be used not just along the aircraft's gliding plane, but slanted against it, like a big kite. This 'camber' allows one to launch at very steep angles, which evens out as the aircraft reaches altitude
 Pneumatic Catapult Launch
Another inclined track, but this time with a pusher carriage which is driven by air pressure, at a constant rate of acceleration. These are very common in the military and professional world for UAVs. This lets one build speed in a very compact area, at the expense of the large catapult rails.
 Winch Launch
A pneumatic winch pulls the aircraft (perhaps attached to a carriage as above) at a constant rate of acceleration. Popular among full-scale sailplanes without electric assist.
 Towplane Launch
A second plane pulls the aircraft to cruise speed (perhaps attached to a carriage as above).
 Runway Launch
Landing gear on smaller airplanes is rarely up to the task of significant obstacles, so a functional landing gear system needs either wheels that are much larger than scale, or a smooth pavement runway.