They exist in the world of sales so we can organize all the opportunities on our daily work. This organization (funnel, pipeline and forecast) allow us to work opportunities to get good leads, keep track of them and follow them and report them to the organization in order to estimate the sales budgets.
Sometimes these terms are confused, but here is a brief explanation of each:
Funnel: The process by which the salesperson reports all sales opportunities although they are not qualified. It is the widest part of the funnel and serves as the initial part of the study of opportunities.
Pipeline: following the Funnel process. Where opportunities are classified and studied so that it can give them the right track and allocate resources in order to turn them into a sale. The pipeline and funnel are often used as one or there are different approaches of how to manage the opportunities with both terms.
Forecast: Once the company decides to attack certain opportunities they should be classified more specifically so you can determine the time of closing, quantity, delivery time and others and should be very detailed with customer information. It must have a high degree of accuracy.
Many organizations use CRM Software to follow up on this classification of opportunities.
In the infographic which can be downloaded at this link; You can get a graphical aid of what the sales funnel is.