The clone and heal tools can be used to get rid of unwanted elements in the image. They are great for small stuff but if you want to get rid of people and trees etc with complex BGs, Photoshop or some other plug-in is your best bet. I wouldn't be surprised however if LR in the next year comes up with an even stronger content remover package. There has been a big improvement from LR5 to LR6.
When you use the CLONE tool you get a 1:1 replacement. That is when you choose to circle an area to be removed, it is replaced with the exact content of the circle selected by LR. You can of course move the "donor" circle to any site you choose. With larger areas you are better doing it in several steps and "blending" over the are to be removed. Moving the FEATHER slider to the right softens the edges of the circle.
The HEAL tool deals more with replacing textures and with small areas this works the best. You can use the circle or click and drag the circle to paint what you wish removed. LR will pick the donor site. Again you can click and drag the donor area around the image to get the best match. You can also hit the / key and LR will make another choice. (Sometimes it is quite comical/frustrating what choice lr makes.) Again it is best to use the smallest area possible to get rid of a spot.
I usually start with the HEAL tool with no/little feather to see how things go and add feather as needed. These take some work to master but are pretty powerful when you get good with them.
Two tricks: 1) suppose you want to get rid of something that is a pretty straight line; a contrail, a branch, telephone wires etc. Select the HEAL tool, make the circle size slightly bigger than the width of the offending line, hold the SHIFT key down, click one end of the line with the circle, let go of the click (not the shift key), and then click the other end. Contrail gone and usually well blended.
Play with it and it will be helpful.
2) Suppose you have a dirty camera sensor. You have a beautiful sunset shot and the sky is full of dust spots. (These come out esp. with HDR images.) With the Clone/Heal tool open, look at the bottom left of the image. There is a VISUALIZE SPOTS box. Check the box and you get a highlight negative that shows all the spots. You can use the heal tool on that screen. If you took several shots of that scene you can copy and paste the spot removal to each image and LR will clean all the spots selected in the first image. (You have to look that one up!)
I couldn't find a really dirty sky so below are two images. The first shows the Visualize Spots box to check; pretend the birds are sensor dust. The second shows the spot screen.
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Uncheck the box to get the image back.