Difference between additive and multiplicative models

Additive and multiplicative models can only be easily distinguished when there is noticeable trend in the time series.

Additive model
For this model, the seasonal, cyclical and residual variation around the trend line are of approximately the same size whether the trend value is low or high.

Multiplicative model
In a multiplicative model, the seasonal, cyclical and residual variation around the trend line are greater when the trend is at high values than when the trend is low.