The blog tagline is the subtitle to the blog title. When you sign up for a WordPress.com blog, its tagline will be the sample text “Just another WordPress.com weblog”. You edit or delete it in Settings > General.
(In Settings > General you can edit the blog title as well. Editing the blog title should not include deleting it, as this is very bad for search engines. You can hide the title if the theme supports that option –see list here– or if you have the Custom Design upgrade.)
The standard tagline shows up in all themes (usually in the header area) except the following ones:
DePo Masthead [see note 1]
DePo Square [see note 2]
Digg 3 Column
Fadtastic [see note 2]
Fleur de Lys
Girl in Green [see note 2]
K2-lite [see note 3]
Neo-Sapien [see note 3]
Shocking Blue Green
Silver is the New Black
The Journalist v1.3 [see note 2]
1. DePo Masthead: doesn’t display the standard tagline, but you can add one under Appearance > DePo Options: Author Box Text.
2. DePo Square, Girl in Green, The Journalist v1.3: tagline displayed in the sidebar (under “About” or “The Author”) if you don’t activate any widgets. Same thing for Fadtastic, if you don’t activate any widgets in the second sidebar (erroneously labeled “Bottombar”).
3. K2-lite, Neo-Sapien: no blog title either.
4. In Under the Influence the tagline can be hidden instead of deleted (Appearance > Theme Options: Description).
5. In themes that don’t display the tagline, you can write a substitute in a Text widget or in the newly introduced Gravatar widget.