Na página de manual:
search This is used to find packages when you know something about the package but aren't sure of it's name. By default search will try searching just package names and summaries, but if that "fails" it will then try descriptions and url.
Yum search orders the results so that those packages matching more terms will appear first.
You can force searching everything by specifying "all" as the first argument.
Normalmente, o yum pesquisa em
- o nome
- o resumo
Com search all , você pode forçar a pesquisa em todos os campos (por exemplo, na descrição também)