What is the difference between membership and participation in the UN?

In the context of the Charter, membership and participation in the UN are different.

UN Charter, Chapter II, Articles 3-6 concern Membership in the UN. Only states can be members of the United Nations.

Membership in different organs of the UN varies.

• Articles 31-32 of the Charter concern participation in the Security Council by states.
• Articles 69-71 of the Charter concern participation in the Economic and Social Council by states, specialized agencies and non-governmental organizations. 
• In addition, the General Assembly has an established practice of granting observer status to non-member states and intergovernmental organizations, see decision 49/426 in A/49/49 (Vol. I), page 341. Entities with observer status receive "a standing invitation to participate as observers in the sessions and the work of the General Assembly" (A/INF/73/5).

To research these matters further, consult secondary sources about the organ or the article of the Charter. For example, the Repertoire of the Practice of the Security Council has a section on the Council's role in membership as well as section on participation in the Council.