7types of crystals:
On the basis of axial length in x,y, zdirection, and interfacial angles between them,unit cell can be classified into seven types which are called Seven Crystal System.
7 Crystal Habits

Axial Length

Interfacial Angles

Cubic

a = b = c

α = β = γ = 90˚^{}

Tetragonal

a = b ≠ c

α = β = γ =90˚^{}

Orthorhombic(Rhombic)

a ≠ b ≠ c

α = β = γ =90˚

Monoclinic

a ≠ b ≠ c

α = γ =90˚ , β ≠ 90˚

Triclinic

a ≠ b ≠ c

α ≠ β ≠ γ ≠ 90˚

Hexagonal

a = b ≠ c

α = β = 90 ˚ , γ =120˚

Rhombohedral

a = b = c

α = β = γ ≠ 90˚

Where a,b,c is the length of x,y and zaxis respectively.
α,β,γ is the interfacial angle between xaxis and yaxis, yaxis and zaxis, zaxis and xaxis respectively.
Note:
Most symmetric crystal Cubic Crystal
Most unsymmetric crystal Triclinic Crystal
To remember this crystal system we have derived a formula and i.e.
CUTE OUR MOTHER
CUCubic
TEtetragonal
OUROrthorhombic
MOMonoclinic
TTriclinic
HEHexagonal
RRhombohedral