A Control is a UGen that can be set and routed externally to interact with a running Synth. Typically, Controls are created from the arguments of a SynthDef function.
Generally you do not create Controls yourself. (See Arrays example below).
The rate may be either .kr (continuous control rate signal) or .ir (a static value, set at the time the synth starts up, and subsequently unchangeable). For .ar, see AudioControl
SynthDef creates these when compiling the ugenGraph function. They are created for you, you use them, and you don't really need to worry about them if you don't want to.
For a more concise combination of name, default value and lag, see NamedControl
adds control names to the SynthDef.
What is passed into the ugenGraph function is an OutputProxy, and its source is a Control.
The main explicit use of Control is to allow Arrays to be sent to running Synths:
Inside SynthDefs and UGen functions, symbols can be used to conveniently specify control inputs of different rates and with lags (see: NamedControl, and Symbol)