A Lie group is simultaneously a group and a smooth manifold e.g.or a circle  which is a specific kind of geometric object. The circle and the sphere are examples of smooth manifolds. Finally the algebraic structure and the geometric structure must be compatible in a precise way.
Informally, a Lie group is a group of symmetries where the symmetries are continuous. The symmetries of a circle are the rotations and reflections in a line through the origin together with the identity and inversion. The rotations may be continuously deformed onto each other within the group, and so might the reflections. This is in contrast with the triangle, which has only three distinct rotations and three distinct reflections.
Lie groups were introducing in the solving of equations. There are many techniques for solving equations, one of which is to make a change of variables which makes the equation simpler and easier to solve, with maybe one variable dropping out of the equations. This occurs because of an underlying symmetry of the equations, and underlying this symmetry is a Lie Group.
Examples:

Euclidean spacewith ordinary vector addition as the group operation.

The circle groupconsisting of rotationsunder addition.

The groupof invertible matrices (under matrix multiplication) is a Lie group of dimensioncalled the general linear group. It has a closed connected subgroupthe special linear group, consisting of matrices of determinant 1 which is also a Lie group.

The orthogonal groupconsisting of allorthogonal matrices with real entries is an dimensional Lie group. This group is disconnected, but it has a connected subgroupof the same dimension consisting of orthogonal matrices of determinant 1, called the special orthogonal group (forthe rotation group).

The unitary groupconsisting ofunitary matrices (with complex entries) is a compact connected Lie group of dimensionUnitary matrices of determinant 1 form a closed connected subgroup of dimensiondenotedthe special unitary group.

Spin groups are double covers of the special orthogonal groups, used for studying fermions in quantum field theory (among other things).

The symplectic groupconsisting of allmatrices preserving a nondegenerate skewsymmetric bilinear form on(the symplectic form). It is a connected Lie group of dimension

The 3sphereforms a Lie group by identification with the set of quaternions of unit norm, called versors. The only other spheres that admit the structure of a Lie group are the 0  sphere(real numbers with absolute value 1) and the circle(complex numbers with absolute value 1).