Andreas
You might guess that economic modelling is a wide subject area - but basically it involves any quantitative approach that seeks to model economic behaviour. Economic modelling is also frequently referred to as "econometrics". So, predictions about the performance of the macro economy (interest rates, GDP etc) are estimated using an economic model that represents the behaviour of the main economy at large. Another example could be efforts to model / forecast market size estimates for a particular industrial / retail sector, or indeed a firms performance within it. The techniques used are also varied, but are often statistical such as multiple regression or time-series (see ARIMA) based.
hope this helps
Iain
