Reentry (technically atmospheric entry) is when anything enters a atmosphere. Specifically, reentry is when something launched from an atmosphere reenters any atmosphere.
During reentry, the air around the spacecraft is heated to plasma, due to compression and friction. This makes for beautiful videos:
When you want a spacecraft to survive reentry, heat is the largest problem. G-loads are a problem, but that's a function of craft aerodynamics and entry angle.
There are many ways to deal with the heat of reentry, here's a couple of the most common ones.
Ablation cooling:
This is where, effectively, you just use the heat of reentry to "ablate" or vaporize a material. Much of the cooling effect comes from the cooler boundary layer created by the gases created from pyrolysis, or decomposition of material at high temperatures without oxygen, of the ablative material. It was first proposed by Robert Goddard in 1920.
The materials used for ablative cooling vary, but can be as simple as treated cork.
Heat sink cooling:
Heat sink cooling, is simply, making the heat shield out of high-thermal-conductivity materials, so that it efficiently moves and stores the heat away from the fragile payload. This was used on the Mercury Redstone suborbital craft with a beryllium heat shield.
The space shuttle used another type of heat sink, reinforced carbon-carbon, or RCC. The biggest problem with this method is the weight.
Radiative cooling:
The space shuttle's black tiles on the lower half of it, known as HSRI, are radiative heat shields. They work by radiating heat back away from the shuttle very efficiently, and by not internally transferring heat efficiently. It is so good at heat radiation and so bad at thermal conductivity that you can hold one side of a HSRI tile while heating the other side with a blowtorch and not get burned.
However, some heat always soaks through, and so the space shuttle always must be hooked up to ground cooling equipment after landing to avoid damage to the aluminum frame.
Active cooling:
This is where you circulate a cooling fluid through the heat shield, or spray a fluid onto the surface of back of a heat shield. The complexity and weight make this less than perfect for most applications.
Transpiration cooling:
Like ablation cooling, this uses a boundary layer to insulate the payload. However, the boundary layer in transpiration cooling is made out of a fluid injected through many small holes in the heat shield.
For more information, and a history, take a look at this ebook: http://www.nasa.gov/connect/ebooks/coming_home_detail.html
I haven't finished reading it yet, but it looks interesting.
