What exactly causes clicks and pops on vinyl?

The above comments pretty much explain why you hear clicks and pops and sometimes a continuous "swish" when you play a record. But it is wrong to attribute it all to careless manufacture and sloppy user practices. The origin of the problem really lies in the technology itself. Recording of sound by mechanical grooves engraved in wax (by Edison) or vinyl today requires incredible precision, and the result is fragile and subject to contamination. Any technology which is so dependent on perfect execution is undesirable. Good technology works well even when you abuse it.

The clicks and pops were never an issue when records were played with acoustic gramaphones (those things with big horns) or early electronic players. The sonic quality was so bad for other reasons, that the clicks and pops were the least of the worries. Today the playback equipment has vastly improved, and every little defect in the groove is faithfully reproduced. Also our expectations regarding sonic quality are much higher. We have taken Edison's invention to its ultimate limits, and the defects which reamain are inherent to the technology.

An analogy might be the typewriter. Over more than a hundred years typewriter machines were vastly improved, perhaps culminating in the IBM Selectric with the type ball instead of levers. Typewriters could go no further. Then came word processing on the computer. The old mechanical character striking device was obsolete in a decade.

Sic transit gloria.
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Acoustat6...I am not a Trekie, so I never heard the Scotty/Spock statment. But it's so true.

This axiom was promulgated back in the 50's by a Japanese engineer, Tagushi, who was a disciple of the great American QC expert Dr Edward Deming. Post war Japanese industry, notibly Toyota and Sony, had adopted the philosophy, and look what it did for them. In practice I learned this lesson in my work on missile guidance systems. Back in the 60's our electronics had lots of precision components, many selected-value components, and even a few pots. Over the years, as performance requirements on the GS tightened up it became evident that, cost asside, it was simply impossible to meet them by using more and more precise (exotic) components in the circuitry. Today it is a requirement on the design of the GS that it not rely on components of unusual precision, and have no selected components or pot adjustments. This puts an additional burden on the designer, but in the end it results in a system having superior performance, and reliability.