open list when cell in column gets focus and append term in list to text being entered

Hi, I have columns that contain text, each column has a list of terms that are related to the text in the cells of that column.
What I would like/need your help to do is:
1. When a cell in one of the columns gets focus a list of the relevant terms for that column will open - and remain open till a cell in another column gets focus.
2. When the user enters text in the cell he can click a term in the list and it will be appended to the text.

Any vba snippets or relevant links much appreciated
Thanks
David

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Hi Tim, Thanks for this, I'm not to bright when it comes to VBA, but from what I understand your checking for a change of value in the cell and appending the new value or replacing the old value, but I can't see any reference to opening a list or or selecting terms from the list - is that correct?
I want to embed the selected term into the text I'm not creating a list from the terms.

Thanks
David