A Forecast or Straight Forecast or Computer Straight Forecast (or CSF for short) are all different terms for the same bet. It is where you pick two selections in the same event in the order in which you think they will come 1st and 2nd.
A forecast bet generally applies to horse racing and dog racing. A Forecast is a bet where you select the first two horses (or dogs) home in the correct order.
A forecast is usually returned is to a one-unit stake. For example: If the forecast return is 11.53 then you are returned 11.53 for every 1 unit of currency you have staked. A forecast return is therefore usually in decimal odds as the dividend includes the stake. So a dividend of 11.53 is actually, in fractional betting odds, 10.53/1.
In UK horseracing, the dividend you receive when you win is the official industry forecast return, which is based on an equation taking into account the odds of the horses involved, the number of runners in the race and many other factors.
In Tote or Pool betting the forecast dividend returned for your win is dependent on how much money was bet into that pool and how many people got the winning forecast. The less people who got the winning dividend then the more the dividend would be.
Sometimes, particularly in general sports betting, bookies will offer their own odds on a forecast. They may have special forecast bets in say the Tour de France (a bicycle race in Europe) or a Formula 1 Grand Prix race. In these types of events the bookies will give all the likely options of who could come 1st and 2nd and they would offer fixed odds on each possible forecast.
What is a reverse forecast?
It is two straight forecast bets i.e: 1st horse selected to beat 2nd horse selected and 2nd horse selected to beat 1st horse selected.
So you choose two selections and it is two separate forecast bets.
What is a combination forecast?
This is the same as a reverse forecast. A combination forecast is all possible forecasts in all-possible orders.
For example: If you choose three selections in an event there are SIX different forecast bets if you bet on all combinations of straight forecast among those three selections.
So a combination forecast is taking more than two selections and have all possible straight forecast combinations with them.
If you stake £1 you are actually placing 6 straight forecast bets, costing a total of £6, using your three selections. All possible Straight Forecast combinations are covered and you will win if two of your three selections come 1st and 2nd in whatever order that happens. Because if that does happen then one of your six forecasts has been landed.