How to add your Company Logo  in salesforce home Tab:

Login into your salesforce account,

upload the image under documents tab

• Go to yourname --> setup -->appsetup -->Customize-->homepage component 
• Under the custom component click New and then click next 
• Give the Name as Logo and select the type as image, click next
• Click the insert image button and select the image/logo file
• Then click save

Now we are successfully uploaded logo into salesforce, next step is we need to enable the custom component in the home page layout

• Go to yourname --> setup -->appsetup -->Customize-->homepage layout
• Click edit the layout
• Then select our component after that click next
• Move our custom component in the top of the left side column
• Then click the save button  

That's it, We are done! Company logo is added into your salesforce account click the Home button.

Custom Label: 

These are custom text value that can be accessed from Apex classes or Visual force pages. These values can be translated into any language Salesforce supports. 

Custom labels enable developers to create multilingual applications by automatically presenting information (for example, help text or error messages) in a user’s native language. 

Limitation

You can create up to 5,000 custom labels for your organization, and they can be up to 1,000 characters in length.

How to access custom label in visualforce page:

{!$Label.demolabel}

How to access custom label in Apex classes:

System.Label.Label__Name;

For In my example:

System.Label. demolabel