TBI is simple and reliable. Its definitely not the most fuel efficient or powerful system out there, but it won't let you down. If it breaks, its pretty easy to fix. You have 2 injectors that mist fuel into the throttlebody where the intake manifold basically pipes the fuel and air into the cylinders from there.
TPI is much more complex and it relies on a few more sensors to run properly. You have as many injectors as you do cylinders. The theory is that as each cylinder needs fuel, it gets its own personal mist of fuel from its injector, therefore allowing for better fuel consumption and better milage. Acceleration and power theoretically should be higher than a TBI system as well.
Converting a TBI system to a TPI system has been done lots, but it is quite a process. There is different wiring involved, a different computer, different intake manifold, different air filter system. Etc. You pretty much want a TPI doaner vehicle nearby unless you want to invest some mega bucks into this project.