how does wash sale work?

my understanding is as follows:
if I sell a stock that has incurred lossses and dont buy the SAME security again for next 30 days, then it is eligible for tax write off. I see some contradictory information on the internet that If i buy any security related to the sector within 30 days, wash rule will be applied.
eg. i have losses with exxon. i sell exxon. on 15th day, i buy Chevron. does my losses on exxon eligible for tax write off?