What is time-cost behavior of projects?

All projects have a goal: something they are trying to achieve. To achieve the goal will take a certain amount of time and need certain resources. These resources have a cost, e.g. people get a salary,machines have wear & tear or need to be hired or purchased. Sometimes you can shrink the time - i.e. make a project end sooner - by increasing the cost, for example by hiring more people or renting faster machinery. Sometimes you can shrink the cost by reducing the feature set; this would not increase the cost by would decrease the time.