I need some explination or the name of this so I can do it myself. Maybe if you could provide also with what I could achieve this would be greatly appreciated:
That is layer blending (possibly with a mask). It's also not particularly well done in my opinion. The easiest way to do that particular effect would be to put the background (the sunset) on a layer beneath the photograph of the two people. Adjust the layer blending to be around 50% or so and then use the eraser to remove the top photo wherever you only want to see the underlying image (which results in it being transparent there.
This technique will work with either Photoshop, GIMP or just about any other layer capable photo editor with Alpha (transparency) support.
It is also possible to get the same thing by setting up a third layer that has a black and white image that covers the area that you want to have blended and then using that as a mask, but that's more complicated to do, varies more by application and isn't necessary for this limited level of blending.