[Q] Noob question topic

I know it sounds odd after so many days of themer on XDA, I am asking it now.

So, what is this themer actually ?

1. Launcher with immense themes (dont want to dig how many)

2. A framework which doesnt conflict with any launcher (for me xperia launcher v3.0)

I am asking this because I dont want 2 launchers in my lightweight "lg p970"
.
And yes, I searched many a times but there is no info about what it is, just everywhere themes, themes, and themes.

Thank you