Could an `analogWrite` command and its variable output values be used to power multiple leds with same intensity?
Single LEDs like the standard 5mm leds work from low voltages such as 1.7 - 2.1v, so can be directly driven via an Arduino pin (through a resistor to prevent excess current draw which would damage the pin). 

LED strips such as automotive LED strips (12v) require the use of a relay if you are simply switching the LEDs on and off. You can't run the relay directly from an Arduino pin, you will need to use a transistor to act as a buffer between the relay and the Arduino to prevent damaging the Arduino. Instead of the motor and diode in the below diagram, you connect your 12v led strip.



If you want to control intensity, you will need to use a transistor, or a mosfet (mosfets are easier). You can then use AnalogWrite to control the intensity of the LEDs via the MOSFET. You will need to connect the control pin for the MOSFET to one of the PWM-capable pins on the Arduino (the pins which have the ~ mark next to them), otherwise you will only be able to switch the MOSFET on and off, similar to the Relay. This site has a good guide on how to do this with a MOSFET. It also has a guide on how to use a transistor, but I can't include the link yet due to not enough reputation points. It shows both the code needed, and how to connect the components to the Arduino.