The term tricyclic refers to the chemical structure of this family of medications. The TCAs were the first known antidepressants, and their immediate effect on the brain is with the neurotransmitters serotonin and norepinepherine. Some act more strongly on one than on the other and some affect both quite strongly.
The TCAs, which were once widely prescribed, have been largely replaced by the safer SSRIs, SNRIs and other more recent additions to the growing list of available antidepressants. Through their effect on the heart, TCAs can be highly lethal in an overdose, essentially they slow the electrical conduction in the heart, and in high enough doses will cause cardiac arrest.
TCAs also carry a heavy side effect burden that includes weight gain, constipation, blurry vision, sedation, urinary retention, and mental slowing. In bipolar disorder TCAs are well known to trigger manic, hypomanic, and mixed episodes. They are rarely used, but when someone is faced with a severe depression that has not responded to multiple other medications, a TCA might be considered.