Lie algebras are defined like vector spaces with a binary operation , bla, bla, bla. The result of the bracket has to be expressed as linear combinations with coefficients in , of course. Some Lie algebras are finite dimensional and other infinite dimensional.
Some Lie algebras are solvable, in the sense that the derived series arrives at 0 in a finite number of steps. As far as I know, they can be finite or infinite dimensional.
An example of an infinite dimensional Lie algebra: vector fields on a manifold . If we fix a distribution in , there is a (possibly infinite dimensional) Lie subalgebra consisting of all the symmetries of the distribution. Remember: they are vector fields such that