Using the Maxim MAX1873
This charger has what's commonly called as power path. Basically, the current coming into the charger is split between powering the system and charging the battery. The actual split depends typically on system current requirements and some settings in resistors. 

The System load is the physical system you want to power (microcontroller, etc). DO NOT connect the battery to this terminal. Doing so could damage the battery. The battery has its own dedicated terminal called BATT.

Power Path allows the system also to switch from being DC powered to battery powered if the charger supports it.