This post, sourced from KB2434864, lists frequently asked questions (FAQs) about download and installation of apps in the Windows Phone Marketplace.
Windows Phone Marketplace FAQ
How do I install a WP app?
When you purchase a WP app, it will download and install automatically for you.
If you remove an app and want to reinstall it later, you will need to buy or install the app again. You will be notified if you have already purchased the app and prompted to confirm reinstall.
Can I share apps with my family and friends?
No. Applications you purchase are only licensed to you.
How do I remove an app from my Windows Phone?
Tap and hold the app icon. From the pop-up menu select uninstall.
How can I keep a backup copy of my apps?
The marketplace does this for you automatically. If you accidentally remove an app from your phone, you can reinstall it by selecting the app in the catalog and selecting buy or install. You will be notified that you have already purchased the app and prompted to confirm reinstall.
I see that there are updates for the apps I’ve purchased, how do I download them?
On your phone, click Updates.
Do updates for apps cost me money?
No. Updates are free of charge.
Is there a limit to how many devices I can download to?
Yes. According to the licensing terms, customers are allowed to download an application to 5 phones at a time that use the same Windows Live ID.