Q:

15.04 is avaliable to update to. Will I lose my data?

Will i lose all my data and applications after upgrading to 15.04 from 14.04 LTS ? My system shows a new update is available - 15.04.

A:

Upgrading in place will not be a guaranteed loss of data - usually, it will not remove your data unless something horribly wrong happens.  That being said, however, you should always back up critical data that you don't wish to lose before upgrading.
Note that this upgrade is optional and you do not need to upgrade to 15.04 if you do not want to.  14.04 is still supported for a while.

About 14.04 -> 15.04 direct upgrade path:
I checked with the Release Team - specifically bdmurray in the #ubuntu-release channel on Freenode IRC. There is indeed a supported upgrade path from 14.04 to 15.04 directly, skipping over the EOL 14.10 release.  This is documented on this bug, Bug #1497024 on Launchpad. It is also only available under the following condition.

if "Prompt=normal" in /etc/update-manager/release-upgrades

However, this path is potentially broken right now as a result of a bug.  You may wish to consider NOT using this upgrade path.

A:

Okay, first things first, this update is 100% optional. 14.04 is supported untill 2019 so you have another 3.5 years before you need to update.
Secondly 15.04 is only supported untill January 2016, so you will need to update to 15.10 pretty soon. You can't update to 15.10 directly, so you will have to upgrade three times - once to 14.10, then to 15.04 then 15.10.
In April 2016 (6 months) Ubuntu 16.04 will be released. This will also be supported for 5 years - and will have all the features of Ubuntu 15.04 and 15.10. It might be sensible for you to wait for then before updating.

If you do decide to update then make sure you have backed everything up - I'd recommended everything you can, even if you don't think you need it. If it's still on your hard drive, you should back it up.
While the upgrade should preserve everything (applications, preferences and permissions), it's not guaranteed. If something goes wrong (e.g. power cut) you want to have your data.
I personally recommend Crash Plan for this - it's very reliable and it's free.

