From Open Energy Information
Typically used by designers, this software allows the user to input project construction and internal energy sources to calculate peak loads for the specific building to determine system design parameters for equipment components. Separate software is available for residential (RHVAC) and commercial (CHVAC) building types along with other (separate) software packages for: electrical, lighting, fire, and plumbing disciplines in addition to various other software packages used in buildings.
Inputs and Outputs
Inputs include: CHVAC is very easy to use and includes a specific "dropdown" help window for virtually every input, including a list of choices where appropriate. The program performs extensive checks on the data you enter and alerts you to any problems that it finds, such as a wall having too much window area, or a leaving coil temperature for an air handler outside of the expected range. Five types of data are requested: general project data, outdoor design data, building material data, air handler data, and specific zone data. The general project data includes the project and client name, designer, building opening and closing hours, internal operating load schedules, and any desired safety factors. The outdoor design data includes the summer and winter outdoor design conditions (automatically looked up for you if a city reference is given) and the desired ventilation and infiltration rates. The building material data includes the definition of master building material types for roofs, walls, partitions, glass sections, and exterior shading. A user defined material library is available for saving the data on common material types. The air handler data includes the fan and terminal type, the desired heating and cooling supply air temperatures and data for duct heat gains and losses. The zone data includes the zone name, floor length and width, number of people, equipment watts, lighting watts, external shading data, and specific roof, wall, partition, floor and glass data. 
Outputs include: CHVAC provides several different types of reports which can be selectively previewed on screen or printed. The reports are: General Project Data, Air Handler Input Data, Zone Input Data, Detailed Project Zone Loads, Air System Zone Summary, Total Building, Air System, and Zone Load Profiles, Air System Total Load Summary, Air System Psychrometric Analysis, Psychrometric Chart, Overall Building Envelope Report, Pie Charts, Bar Graphs, and the Total Building Load Summary. Virtually all calculated data can be exported to a comma delimited text file that you can then import into your favorite spreadsheet program such as Excel. 
Used in sizing loads on building from internal gains as well as from external sources (sun, weather generally)
This is one of a number of commonly used load sizing programs, fairly easy to use (for a design professional), graphical representation is an add-on to the base (RHVAC, CHVAC) program