it's just Engineering/ Mathematics/ Physical Symbol that show that we reached the maximum Condition of something.
Uses in Engineering:
1. if we have oil tank contain 1000 Litter and the pump didn't stop at its limit and kept pumping oil
In the tank,
Well then we can say that the 1001 is the infinity for that tank
but hey we can't say that infinity is mean 1001 for the tank
Because the 1002, 1003, 1004, 1005, 1006 litters .....etc, is the infinites of that tank ....... So we can't determinate the infinity
2. If we decided to measure the tall of student college we can make calibration that the max of the student tall measurement tall is 3 meters
so the 3.1, 3.2 is the infinity
3. If we are designing a Product of course as engineers we test the material selection, the stress and the strain during designing process so if the product failure at the force of 5 Newton so the 5, 7, 8 is infinity for the product
PS:
The infinity can be Zero in another application look to Example 1 the oil tank and to example 2 the Student Tall the infinity in the second example which is 3 can be almost zero in the tank example
So the infinity is depend on the application and max value of it
From that we can say that the infinity is not a number it’s just a way to express the maximum statue of the application
Also that's why when we say
∞ - ∞ = “Indeterminate form”
Because we don’t know which ∞ is bigger than the other
But…
∞ + ∞ = ∞ because both of them is ∞ so the sum must be in the ∞
Also do you know that Parallel line is intersected in the infinity 
We Assumed that as we don’t know where’s the Infinity
Read more:
Indeterminate form
http://en.wikipedia.org/wiki/Indeterminate_form
Infinity @ Wikipedia
http://en.wikipedia.org/wiki/Infinity
