Prisms
Go to Surface Area or Volume.
A prism is a solid object with:
 identical ends
 flat faces
 and the same cross section all along its length !
A cross section is the shape made by cutting straight across an object.
The cross section of this object is a triangle ...
.. it has the same cross section all along its length ...
... so it's a triangular prism.
Then imagine it extending up from the sheet of paper ... 
No Curves!
A prism is a polyhedron, which means all faces are flat!
No curved sides.
For example, a cylinder is not a prism, because it has curved sides.
Bases
The ends of a prism are parallel
and each one is called a base.
Sides
The side faces of a prism are parallelograms
(4sided shapes with opposite sides parallel)
These are all Prisms:
Square Prism:  CrossSection: 
Cube:  CrossSection: 
(yes, a cube is a prism, because it is a square all along its length) (Also see Rectangular Prisms ) 

Triangular Prism:  CrossSection: 
Pentagonal Prism:  CrossSection: 
and more!
Regular and Irregular Prisms
All the previous examples are Regular Prisms, because the cross section is regular (in other words it is a shape with equal edge lengths, and equal angles.)
Here is an example of an Irregular Prism:
Irregular Pentagonal Prism:  
CrossSection  
It is "irregular" because the crosssection is not "regular" in shape. 
Surface Area of a Prism
Surface Area =  2 × Base Area 
+ Base Perimeter × Length 
Example: What is the surface area of a prism where the base area is 25 m^{2}, the base perimeter is 24 m, and the length is 12 m:
Surface Area  = 2 × Base Area + Base Perimeter × Length  
= 2 × 25 m^{2} + 24 m × 12 m  
= 50 m^{2} + 288 m^{2}  
= 338 m^{2} 
(Note: we have an Area Calculation Tool)
Volume of a Prism
The Volume of a prism is the area of one end times the length of the prism.
Volume = Base Area × Length
Example: What is the volume of a prism where the base area is 25 m^{2} and which is 12 m long:
Volume  = Area × Length  
= 25 m^{2} × 12 m  
= 300 m^{3} 
More About The Side Faces
The side faces of a prism are parallelograms (4sided shape with opposites sides parallel)
A prism can lean to one side, making it an oblique prism, but the two ends are still parallel, and the side faces are still parallelograms!
But if the two ends are not parallel it is not a prism.