Everything wears out eventually and that is true of these cards as well. What wears them out is the read write cycle and to a much lesser extent handling.
A very simple analogy would be a piece of paper which has two ways that it gets worn out.
First you can write on the paper then erase that information and write on it again; eventually you wear a hole in the paper. As humans we can read the paper without damaging it, but if we expose to strong light like scanning we do wear it out and it will get harder to read.
The same thing happens in the memory card but you have some major advantages.
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You can write,read and erase on any part of the memory at least 100,000 times.
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The controller on the card will "spread" your activity all over the paper so you wont wear out one spot.
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The controller also can block off bad or worn out sectors and not use them anymore.
This means that the card will show less capacity as it wears out. This is easy to check by using the properties feature and reading it like a drive; if you see the available space number dropping that means your card is wearing out.
Secondly as you handle a piece of paper the edges get frayed and eventually it tears. For an SD card that is kept dry and handled with reasonable care this really isnt of an issue. These cards are incredibly robust and can survive years of normal handling. The connectors on a SD card are rating for 10,000 insertions.
