To run a simulation, not only must the circuit be defined, but also the type of analysis to be performed. There are six different types of analyses: linearized small signal AC, DC sweep, noise, DC operating point, small signal DC transfer function and transient analysis. Precisely one of these six analyses must be specified when you run a simulation.
Whereas the circuit topology is typically schematically drafted, the commands are usually placed on the schematic as text. All such commands start with a period and are called "dot commands".
.AC -- Perform an Small Signal AC Analysis Linearized About the DC Operating Point
.BACKANNO -- Annotate the Subcircuit Pin Names on Port Currents
.DC -- Perform a DC Source Sweep Analysis
.ENDS -- End of Subcircuit Definition
.FOUR -- Compute a Fourier Component
.FUNC -- User Defined Functions
.FERRET -- Download a File Given the URL
.GLOBAL -- Declare Global Nodes
.INCLUDE -- Include Another File
.LOADBIAS -- Load a Previously Solved DC Solution
.MACHINE -- Arbitrary State Machine
.MEASURE -- Evaluate User-Defined Electrical Quantities
.MODEL -- Define a SPICE Model
.NET -- Compute Network Parameters in a .AC Analysis
.NODESET -- Supply Hints for Initial DC Solution
.NOISE -- Perform a Noise Analysis
.OP -- Find the DC Operating Point
.OPTIONS -- Set Simulator Options
.PARAM -- User-Defined Parameters
.SAVE -- Limit the Quantity of Saved Data
.SAVEBIAS -- Save Operating Point to Disk
.SUBCKT -- Define a Subcircuit
.TF -- Find the DC Small-Signal Transfer Function