Confused about ground

I'm a real newbie to electronics and I'm just starting my first project with the uno R3. I have an RGB LED , a mini laser and seven switches, all of which need to be connected to ground. However, I'm confused about whether I can connect inputs and outputs to/from the arduino to the same ground pin? Also, how many components would you say is the maximum for each ground pin?


No limit really as long as total current is below the limit. Just start with some tutorial experiments/projects first.

There are basically two types of grounding techniques.
1- common ground: (this is the most common. all of the driven devices share the uController ground)
2- Isolated grounds: The microcontroller has one ground and the driven device (ie: motor controller)
has a separate ground. The microcontroller drives opto-isolators that have a transparent 5000V dialectric
isolation barrier through which a led drives a photo-transistor. the led shares the uC ground the
transistor shares the motor controller ground. There is no continuity between the grounds. This prevents
the motor controller from adversely affecting the uC. The motor controller may or may not have it's own
opto-isolators. If it doesn't then the pullup resistor on the collector of the transistor pulls the voltage at
the collector to Vcc (ie: 5V dc) . When the transistor turns ON, it pulls that Motor Controller input LOW.
This method results in negative logic. All of your code would have the signals inverted because they are
Active LOW. (a HIGH from the uC, results in a LOW on the Motor controller input). It is a simple matter
to change the code to accomodate this. If you are not driving a motor controller, there is no reason to
use isolated grounds, so you would chose option 1. (common ground; everything shares the uC GND).