A Lie groupoid can thus be thought of as a "many-object generalization" of a Lie group, just as a groupoid is a many-object generalization of a group.
A Lie groupoid is a groupoid where the set $\operatorname{Ob}$ of objects and the set $\operatorname{Mor}$ of morphisms are both manifolds, the source, and target operations $$s,t:\operatorname{Mor} \to \operatorname{Ob}$$ are submersions, and all the category operations (source and target, composition, and identity-assigning map) are smooth.
A Lie groupoid can thus be thought of as a "many-object generalization" of a Lie group, just as a groupoid is a many-object generalization of a group. Just as every Lie group has a Lie algebra, every Lie groupoid has a Lie algebroid.
Eg: Any Lie group gives a Lie groupoid with one object, and conversely. So, the theory of Lie groupoids includes the theory of Lie groups.