Disk Space and Bandwidth
The amount of storage space given to you by your free web host is called the disk space. This is where your website files will be stored - such as images, text, audio and other files used for your site.
The amount of traffic permitted to access as well as leave your website is called the bandwidth. Access is the time when a visitor keys in the name of your website in his/her browser. Leave is the time when a visitor gets info from your site like when viewing a photo, downloading a file or listening to audio.
For instance, when you website contains lots of graphics like photos, you will need bigger bandwidth and more disk space.
Website Creator and FTP
If you are new to creating a website, and you do not have any or enough scripting experience, look for a free web host that provides free website creating apps to help you develop your site just by simply choosing a template, then adding your text and images.
File Transfer Protocol or FTP is the protocol to transfer your website files to your free web hosting server from your computer or the other way around. This lets you upload your completed website files like images plus other multimedia files to your hosting server from your computer. Also, it lets you download your files from your web host back to your computer. Every reliable free service should provide 24/7 unlimited FTP access.
Your free web host should be able to offer you email services to let you set up @yourdomain.com email accounts. Aside from a webmail interface (to login and handle emails using your web browser), this email service should come with SMTP access to let you setup and access your @yourdomain.com emails via your mobile gadgets.
Support and Uptime
Always go for a free web hosting service that includes free telephone and email support that you can count on.
Though there is no host that provides 100% guaranteed uptime (issues beyond their control like city power outage), they must give "just-in-case" ways to minimize website and email downtime. Such measures may incorporate 24/7 server plus network monitoring, network configurations plus redundant hardware, as well as readily available backup power supply. Though you may not fully understand these technicalities, check if your chosen free service includes these on their site. Otherwise, inquire about it.
If you are opting for a free web host, you are not expected to pay anything at all for the aforementioned features. However, you might have to accept banner ads in exchange of the pricing. If free web hosting providers force you to include ads on your website, forget about them.