Hi
On Joomla! you will find a huge amount of extensions from 3th party team that develop applications (components, plugin, modules) to work under joomla environment. This means that this apps extend joomla features and gives you the ability create specific content on dedicated environment.
K2 is a component very powerful and flexible, that allows you to create content like blog, news, store (with K2 store or K2Mart), catalogue and so on... all supported with native media plugins.
VM aka Virtuemart is a dedicated e-commerce component.
The blog that you are referring is Joomla! native component to display articles, news or other different types.
Both K2 and VM are very well supported by Joomla! Community and you can use them with the guarantee that they will have support, as long joomla! exists (hopefully forever
)
In our case, we build joomla templates and some extensions to enrich your projects; and we also select some 3th party components to include on our projects. Mostly they are K2, K2 Store, JomSocial, RedShop and Virtuemart components.
It's up to you, but here's a suggestion. Start from ground zero, play and learn joomla.
It's easy when you learn with small experiences. When you have an idea how joomla works, them it will be more easy to understand others.
Cheers