The process of finding a limit is a key part of differentiation and calculus. Many limits are difficult to evaluate and we will only be studying some of the more basic limits.
Limits
Finding the limit of a function at a certain value involves investigating the value of the function as x approaches this value.
This can be done using algebra or by looking at the graph of the function.
For the function f(x) = x + 4
We say

On the graph of f(x) = x + 4


In words, as the value of x approaches 3, the value of the function approaches 7.

To evaluate a limit of this type the xvalue is substituted straight into the function.
Often this is not possible and the function needs to be simplified or written in another form first.
Example Evaluate
The xvalue of 2 cannot be substituted into the function as the denominator would become 0 and division by 0 is not possible. So the function is factorised and simplified and the 
Some limits have to be found as x gets larger and larger, we refer to this as letting "x tend to infinity".
Example
On a graph, this would be the yasymptote of the hyperbola.
As x gets larger the yvalue approaches 3.
Note that as x approaches 1, there would be NO LIMIT as the line x = 1 is an asymptote and the yvalue on either side of the asymptote gets very large or very small.
Algebraically, this is recognised by the expression "non zero over zero" when a direct substitution is attempted.
Continuity
A function is continuous at all points, if the graph can be drawn without lifting your pen off the page!
In other words there are no breaks in the graph.
e.g.
This function is not continuous at x = 4 as f(4) = 7 when
A hyperbola would not be continuous where the vertical asymptotes occur. See the graph of the hyperbola above.
Differentiability
Before we study differentiation, we need to establish when a function can or cannot be differentiated. A function cannot be differentiated:
a. at any holes in the domain.
b. at any point where the gradient is significantly different on either side of the point.
c. where the function is undefined.
Function 
a. 
b. 
c.

Differentiability 
Function is not differentiable
at x = 1 
Function is not differentiable
at x = 1 or at x = 1 
Function is not differentiable
at x = 1 
Continuity 
Function is not continuous
at x = 1 
Function is continuous
for all values of x 
Function is not continuous
at x = 1 
Note A function can be continuous at a point but not differentiable e.g. Graph b. above.