STM32F3 - diffrence between PWM1 and PWM2

Can anyone explain to me wht is the difference between PWM1 and PWM2 in STM32F3 timer configuration? It is mentioned frequently in documentation, however I can't find the difference between these two settings. Code example for setting PWM1 OC mode for timer 8:

Tim_InitStruct.TIM_OCMode = TIM_OCMode_PWM1;
Tim_InitStruct.TIM_OutputState = TIM_OutputState_Enable;
Tim_InitStruct.TIM_Pulse = 512 / 2; // 50%
Tim_InitStruct.TIM_OCPolarity = TIM_OCPolarity_Low;