Bubbles on inputs mean that these inputs go low to activate the gate.

In the low activated AND shown above, both inputs must go low to make the output go high.

In the low activated OR below, A or B going low makes the output go high.

In the low activated NOR circuit, A or B going low makes the output go low.

A long bar above the output indicates that the output goes low when the gate is activated.

The short bars indicate that those inputs must go low to activate the gate.