Astable (oscillator)
Free-running square wave. Discharge runs through R2 only, so the output is high a bit longer than it is low (duty is always above 50% with this classic two-resistor circuit).
f = 1.44 ÷ ((R1 + 2·R2) · C), high = 0.693·(R1+R2)·C, low = 0.693·R2·C.
Monostable (one-shot)
A single pulse on each trigger. The output stays high for one time constant and a bit.
pulse = 1.1 · R · C.