I will provide the answers to these questions with examples in this post.
Reliability is within a reasonable amount of time, which depends on the application type and architecture, delivering the legitimate packets from source to destination.
This time is known as delay or latency and it is one of the packet delivery parameters. Consistency of delay is known as jitter and it is very important for some type of applications such as voice and video, jitter is our second delivery parameter.
I am explaining this topic in deep detail in our CCIE Enterprise Course and Self-Paced Cisco CCDE course.
The third packet delivery parameter is packet loss or drop; especially voice and video traffic are more sensitive to packet loss compared to data traffic.
Packet loss is application-dependent and some applications very drop/packet loss sensitive.
General accepted best practices for the delay, jitter, and packet loss ratio has been defined, and knowing and considering them is important from the network design point of view. For example, for the voice packets, a one-way delay which is also known as the ‘mouth to ear' delay should be less than 150ms.
Reliability should not be considered only at the link level. Network links, devices such as switches, routers, firewalls, application delivery controllers, servers, storage systems, and others should be reliable; also component of these devices needs to be reliable.
For example, if you will carry the voice traffic over unreliable serial links, you may likely encounter packet drops because of link flaps. The best practice is to carry voice traffic over the low latency links, which don't have packet loss and latency. If you have to utilize those cheaper unreliable links such as the Internet, you should carry the Data traffic over them.
But actually whichever device, link or component you choose, essentially they will fail.
Vendors share their MTBF (Meantime between failure) numbers. You can choose the best reliable devices, links, components, protocols, and architecture; you need to consider unavoidable failure.