It is often possible to understand mathematical and physics structures by understanding their symmetries.
A symmetry is something that remains the same (is invariant) when something else changes. When we are analising a problem in maths or physics what we are often doing is looking for symmetries, after all if everything changed and nothing remained constant, not even some function or equation then how could we begin to analyse it?
In group theory terminology the symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation (see this page for a more intuitive geometric interpretation of symmetry).
One of the simplest types of symmetry is mirror symmetry, this is often what people think of when they hear the word 'symmetry'. In mirror symmetry the perpendicular distance from mirror remains the same on reflection. If a shape has mirror symmetry then it will map onto itself when reflected. However there are many other types of symmetry:
Change  What Remains Constant 

reflection in a mirror  perpendicular distance from mirror. 
rotation  axis of rotation, distance to axis, points in plane perpendicular to the axis remain in that plane. 
Object falling in a vacuum.  acceleration 
These symmetries are groups or, to put it the other way round, groups represent symmetries. This allows us to formalise symmetry. For an introduction to groups see this page. One of the great mathematical achievements of the 20^{th} century was the categorisation of groups, here is a very top level overview:
Group Family  Type of Symmetry  Example  

Cyclic  'n' way rotation in finite steps in a plane.  Square can be rotated to four positions  
Alternating  finite step rotational symmetry of the nsimplex in 'n' dimensions.  Tetrahedron in 3 dimensions  
Lie  continuous symmetry

Properties of physics are the same regardless of the orientation in 3D space of the coordinate system.  
Sporadic  ?  ? 
Symmetry Operations
Examples of symmetry operations are translation,rotation and reflection.
An isometry is a distancepreserving isomorphism between metric spaces.
A composite function is the result of applying one function and then applying another function. So if X is an operation and Y is another operation then X*Y is the composite operation.
Note that in order to define a symmetry group we must define distances (metrics) in a space so, for example, Minkowski space and Euclidean space have different symmetry operations.
Symmetry Groups in different numbers of dimensions
space  transforms symmetry groups are some combination of: 

1 dimension  translation 1 dof reflection 
2 dimensional Euclidean space  translation 2 dof: rotation 1 dof:
reflection in x 
3 dimensional Euclidean space  translation 3 dof: rotation 3 dof: reflection in x 
4 dimensional Euclidean space  translation 4 dof: rotation 6 dof: reflection in x 
4 dimensional Minkowski space  