Codex
Just take a look at the WordPress Template Hierarchy.
Templates
You’ll find out that WordPress has three different types of templates, that you can use:
A) For built in taxonomies: Category & Tag
B) For custom taxonomies.
The template engine will search for such templates when you request a category/tag/custom taxonomy archive page.
taxonomy-{$taxonomy_slug}-{$term}.php
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taxonomy-{$taxonomy_slug}.php
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taxonomy.php
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category-{$slug}.php
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category-{$id}.php
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category.php
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tag-{$slug}.php
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tag-{$id}.php
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tag.php
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Overview
The excellent diagramm by our man Chip Bennet.