Tissue is made from pummled trees. Once sufficiently pummled, the wood bits are soaked in a variety of chemicals to produce a fine white paste.
This paste is thined, scented, dried, folded, and packaged. Each tissue sheet is roughly 6 inches by 10 inches rectangular and thin enough to see through.
Any intelligent species can readily determine that this should be a thouroughly silly way of capturing germs, although, it has been remarked
that an advanced civilization wishing to eradicate earth creatures, did in fact, bring the tissue to earth millenia ago for the sole purpose of spreading disease (they brought the common cold with them too, so say the JFK theorists).
One common non-icky use is for cleaning glasses. This is hampered by the invention of the "soothing lotion" containing tissue. This sub-set of tissues is designed to have greasy stuff incorporated into the pummled wood paste that sooths sore red bits that have been wiped up by too many regular tissues. The effect on glasses, however, is downright disasterous. The bits of glasses that should be relatively clear, become basicly opaque from smears of lotion.
Tissues are also frequently used as play-things for children. This is generally accomplished as "paper flowers" or "paper fans" (which REALLY don't work well).