You haven't really asked a question. But i'll assume that you want to know what a base and sub-theme is in drupal.

##TLDR version

A drupal subtheme inherits the template files (*.tpl.php) and assets (javascript, css, images) of the base theme. If you need to modify or change the template files.

###More on subthemes

I think your best bet is to google it. I googled, "drupal subtheme" and found you drupal documentation regarding subthemes:

Sub-themes are just like any other theme, with one difference: They inherit the parent theme's resources. There are no limits on the chaining capabilities connecting sub-themes to their parents. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential.

Source: Creating a sub-theme (drupal.org)

In addition to the node page, if you're more inclined to watch a video this is probably a good bet: Drupal Subthemes (youtube.com)

###Why should you avoid modifying a base theme

• Flexibility: Base themes maintain flexibility. If there is an update, your child theme inherit these changes
• Losing your changes: Modifying your base theme means that when an update to the theme comes along your modifications could potentially be lost
• Maintain simpler code base: The base theme allows you to abstract a lot of functionality and reduce the code base. This means less maintenance in the long run.

###Best Practices with subthemes:

• Copy files from the base theme or use the drush command (when available)
• Utilize subtheme theme hooks to override your theme

You haven't really asked a question. But i'll assume that you want to know what a base and sub-theme is in drupal.

TLDR version

A drupal subtheme inherits the template files (*.tpl.php) and assets (javascript, css, images) of the base theme. If you need to modify or change the template files.

More on subthemes

I think your best bet is to google it. I googled, "drupal subtheme" and found you drupal documentation regarding subthemes:

Sub-themes are just like any other theme, with one difference: They inherit the parent theme's resources. There are no limits on the chaining capabilities connecting sub-themes to their parents. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential.

Source: Creating a sub-theme (drupal.org)

In addition to the node page, if you're more inclined to watch a video this is probably a good bet: Drupal Subthemes (youtube.com)

Why should you avoid modifying a base theme

• Flexibility: Base themes maintain flexibility. If there is an update, your child theme inherit these changes
• Losing your changes: Modifying your base theme means that when an update to the theme comes along your modifications could potentially be lost
• Maintain simpler code base: The base theme allows you to abstract a lot of functionality and reduce the code base. This means less maintenance in the long run.

Best Practices with subthemes:

• Copy files from the base theme or use the drush command (when available)
• Utilize subtheme theme hooks to override your theme

You haven't really asked a question. But i'll assume that you want to know what a base and sub-theme is in drupal.

##TLDR version

A drupal subtheme inherits the template files (*.tpl.php) and assets (javascript, css, images) of the base theme. If you need to modify or change the template files.

###More on subthemes

I think your best bet is to google it. I googled, "drupal subtheme" and found you drupal documentation regarding subthemes:

Sub-themes are just like any other theme, with one difference: They inherit the parent theme's resources. There are no limits on the chaining capabilities connecting sub-themes to their parents. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential.

Source: Creating a sub-theme (drupal.org)

In addition to the node page, if you're more inclined to watch a video this is probably a good bet: Drupal Subthemes (youtube.com)

###Best Practices with subthemes:

• Copy files from the base theme or use the drush command (when available)
• Utilize subtheme theme hooks to override your theme

You haven't really asked a question. But i'll assume that you want to know what a base and sub-theme is in drupal.

##TLDR version

A drupal subtheme inherits the template files (*.tpl.php) and assets (javascript, css, images) of the base theme. If you need to modify or change the template files.

###More on subthemes

I think your best bet is to google it. I googled, "drupal subtheme" and found you drupal documentation regarding subthemes:

Sub-themes are just like any other theme, with one difference: They inherit the parent theme's resources. There are no limits on the chaining capabilities connecting sub-themes to their parents. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential.

Source: Creating a sub-theme (drupal.org)

In addition to the node page, if you're more inclined to watch a video this is probably a good bet: Drupal Subthemes (youtube.com)

###Why should you avoid modifying a base theme

• Flexibility: Base themes maintain flexibility. If there is an update, your child theme inherit these changes
• Losing your changes: Modifying your base theme means that when an update to the theme comes along your modifications could potentially be lost
• Maintain simpler code base: The base theme allows you to abstract a lot of functionality and reduce the code base. This means less maintenance in the long run.

###Best Practices with subthemes:

• Copy files from the base theme or use the drush command (when available)
• Utilize subtheme theme hooks to override your theme