Dot Net Framework query

[#1]

Haven`t got a clue what it is, or what it does for me.!...
so... in my list of installed programs I have the following
Microsoft .Net Framework 3.5 SP1  ....... 101.7Mb    installed on 10/10/2013
Microsoft .Net Framework 3.5 SP1 .........  27.4Mb  installed on  6/10/2013

and....
Microsoft .Net Framework 4.5      ..... 2200.3Mb      installed on 10/10/2013
Microsoft .Net Framework 4.5      ..... ......38.8Mb        installed on 6/10/2013

Seems to me a bit of "overkill"... to install two verssions fo SP1 within 4 days... and at the same time(date) install a later version with two more versions of that on the same dates...
can I uninstall anything? or do I just have to have them all?

[#2]

The trouble with the various .Net framework versions is that they do not guarantee backwards compatibility so in principle you could need all versions.  In practice you may well need none.
.Net framework is yet another programming language.  I have no idea what benefits it brings but you rightly observe that it consumes a huge quantity of updates, which can be very slow to install on older machines 

[#3]

Some programs are written using programming languages and components that are part of Microsoft .NET. You won't be able to run those programs without the .NET Framework installed.
Programs written to use .NET 4 need the version 4 framework.
Programs written to use .NET versions 2, 3, and 3.5 need the 3.5 framework.
If you uninstall them, don't complain later about how long it took to install one of them and 13 updates for it again, if you later install something that needs one of the framework versions.

[#4]

OK.... so, I understand that I "probably" need the 3.5 and the 4.5 versions... what I don`t understand is why there are two installs of each version 4 days apart, and different file sizes... ????

[#5]

dot-net ships with Windows 7 and 8 by default - and updates itself, by default.
.net is a runtime environment - that means it loads, runs and manages programs. It's rather like Java Runtime and like Java, there's also a development version of .net - this is used to write and compile programs in languages like c-sharp and VB.NET, often using Visual Studio, etc
You get the previous .net incarnation of 3.5 because namespaces contained therein are leveraged by the newer 4.5+ one. There's no discrete version numbers with .net - just lots of directories called 'Framework' something.
Why you have little and large versions is possibly the little versions did an in situ update, and then loaded the big ones... or visa versa. The release of dot-net 4.5.1 was last October - which is consistent with your drive dates.
Anyway, you're right to not to delete it.
It's all a bit too System.Net.Mysterous

[#6]

Sorry, Thundercllap.... load of gobbledi gook to me... Wish I never asked   

[#7]

Quote from: ReedRichards

.Net framework is yet another programming language.

No it is not!
.NET can be used by ANY language that supports it but the .NET framework itself is actually a core set of components (such as edit boxes, menus, buttons etc) that you would find in most programs that have been created at MS for any language that supports .NET to take advantage of.
The downside is that if you design a .NET program in any language, your users must have it installed as the components used are not compiled into the final executable. It's a bit like forms that are stored in a .dll file. You need your end user to have the .dll to access the form / pieces of program contained within.
Ultimately you don't really need to know why you need .NET. If a program author says it needs it then it needs it. It is however IMO a PITA.

[#8]

My shoulders still drop when I see .NET in the list of essential updates

[#9]

Me too. I hate .net updates.. thats why like with most other updates i don't bother 

[#10]

@ Thunderclap.... P.S.  I am running Vista Home Premium ! ! ! 

[#11]

Sound to me that some other package on your PC needs dot-Net as a prerequist.
Removing dot-net may abend something else -> and then you'll have to reinstall all 2+ gigs of it again.