Ok so I have been doing a lot of reading and research on this here web hosting business and I have yet to understand some things. Like this dedicated and semi dedicated servers. Which is better? Which can make my life easier?

Well its a boring answer but it always depends on your requirement, semi dedicated servers don't provide you root access and rarely the ability to install 3rd party applications, but they are cost effective solution for high traffic websites.
Dedicated server is an option if you are facing load issues with a semi dedicated option and/or you need to install custom application that your host is not willing to install for you on a semi dedicated server (logically so, coz an application of your use would be useless for other clients on an SDX server, but still eat resources).

As Howard mentioned a dedicated server comes with full root access and is the answer if your semi dedicated option is encountering problems such as load related issues - it's a step up.

Semi dedicated plans are meant for websites that have a lot of traffic/use a lot of resources. However they do not come with full root access which dedicated servers have.

Root access is the main distinctions between the two,However There are other distinctions as well.Try researching what you need and go For what suites you.

Our experiences showed that semi-dedicated, VPS, etc. - whatever you call them- never work and feel like a dedicated server.

How does a VPS not feel like a dedicated server?
It's exactly the same except for space and transfer limitations.

Genesis touch on Root access as being a factor... and correct. But to drill down on the value add of Root, the following question is... how much support do you expect from your data center? if no support is required and you have the cash... go with dedicated and you'll have full control of your box. If you require a bit more hand holding then shared hosting is the only affordable option but keep in mind... their guys have access to your box and mistakes can and will happen leaving you standing in a queue line waiting for tech support to take your call.

Just to clarify VirgoHosting's statement.... VPS and Semi-Dedicated servers ARE different.

Here's the main breakdown;

Shared Hosting - you share with lots of other people
SemiDedicated - you share with limited people
VPS - your own environment, with ROOT access. Usually with 4 or more VPS environments on a server
Dedicated Server - Root access, your own machine. Do anything you want.

If you can't afford dedicated server, its better semi-dedicated than VPS

Depending on your needs. If you want to have your own scripts running (as root) you can't do that in the Semi-Dedicated environment. Same goes for custom modules or compiles of Apache etc. I don't know any Semi-Dedicated that can do this.

Yes, but there are some also offered this, anyway not like VPS you can do a root access.

If you have the cash then I highly recommend a dedicated server (there are some great deals out there so they are are a lot more affordable)

If you don't quite have the cash and you want root access and a fair enough of specs I recommend a VPS.

A semi dedicated plan is just a glorified reseller account (which can be good if it suites your needs).