Check the legend on your sectional to see when the restricted airspace is
active. Few restricted areas are active all of the time. You can fly through
when it is not active. You also can fly through with permission or vectors
from ATC, or you can call the local controlling agency and find out
procedures for a particular restricted area.

Usually restricted areas are there for your safety -- something is going on
in there that is extremely dangerous to aircraft. Permission to fly through
restricted areas is routinely given when circumstances permit. Areas where
air traffic is permanently not allowed for security reasons are usually
prohibited areas. Prohibited areas are rare.

Some areas have "temporary" restricted areas around them. These used to be
used for controlling traffic around major sporting events or news events and
around other areas attracting intense aerial activity, such as forest fires.
The new TFRs are more or less permanent and are used much the same way as
prohibited areas are, except there are numerous exceptions allowed for
traffic to fly through them.