A lot of old houses have fuses and regular two prong outlets. GFCI sockets are a boon to a no fuss DIY rewiring of the house to make it much safer. A nice description of GFCI sockets is given here (In general stackexchange is awesome and I would recommend joining). A nice description, with pictures, of how to rewire a GFCI socket is given here.
The GFCI protects against ground faults. This means that if there is some problem with the equipment you plug in and any part of the wiring becomes, effectively, exposed, you are protected against getting a shock from that. However, nothing can protect against a deliberate touching of the live and neutral lines.
GFCIs are cool because you can use them for two prong outlets (with no ground). You still need three prong outlets for surge suppression - the surge suppressor redirects the surge to ground (which is basically a fat, extremely low resistance line, allowing the surge to flow past the equipment and into the ground beneath the house).
The typical recommendation is to have GFCIs for any outlets with any water nearby - kitchens, bathrooms, basements and the outdoors. They are more expensive than regular outlets, but there is no harm in replacing all outlets with GFCIs.
Note: Equipment you will need when replacing outlets
- Electrical tape
- Non-contact tester
- Electrical tester
- Insulated screwdriver
- Outlet tester