There are mainly two mechanisms that send radiation back into space: reflection and emission. In the first case, electromagnetic waves coming from the sun are simply reflected, with the same high frequency. This is due mainly to clouds, that in this respect contribute to cooling the earth's temperature. The fraction of the incoming light that is reflected by a planet (or any other object) is called albedo. For the earth, it is about $0.3$, which means that about $30\%$ percent of all the energy from the sun is already reflected into space by reflection (this is a rough estimate, different parts of the earth have different albedos: oceans and forests have a lower one, poles a higher one and so on).
What is left can be absorbed and then re-emitted with a lower frequency as thermal radiation (typically in the infrared). As far as emission is concerned, we have to take into account the effect of the atmosphere (for example, the ozone layer subtracts energy, absorbing the higher frequencies), of the surface and of clouds (apart from reflection, they also absorb the radiation emitted by the surface; part of it is emitted again toward the surface and part out into space - the mechanism is complicated and typically depends on the difference in temperatures between the top and the bottom of a cloud, the type of particles in the clouds and so on). The net effect is that the atmosphere raises temperatures on earth (without the atmosphere, the average temperature would be $-19$ degrees celsius or so). But in the long run (apart from what Ron mentioned), all the energy is re-emitted into space. This may seem paradoxical, but as pointed out for example by R.Penrose, what really matters for life is that the sun is a hot spot in an otherwise dark sky (it is a source of low-entropy energy: yellow energetic photons are absorbed by the earth and emitted in the infrared in larger numbers; this means more degrees of freedom, more volume occupied in phase space or more entropy).