Tier 5 (Temporary Worker - Charity Workers) category is for people coming to the UK to do voluntary (unpaid) work for a charity. The work must be directly related to the work of your sponsor's organisation.
If you are coming to work temporarily in the United Kingdom as a charity worker, you should only be doing voluntary activities and not paid work. You should intend to carry out fieldwork directly related to the purpose of the sponsoring organisation.
In assigning a Certificate of Sponsorship, the sponsor will have guaranteed that you:
intend to undertake voluntary fieldwork directly related to the purpose of the charity;
will not be paid or receive other payment for your work;
will will not take up a permanent position; and
will comply with the conditions of your permission to stay and leave the United Kingdom when it expires.