What counts as close contact?

Someone who was within 6 feet of an infected person for a cumulative total of 15 minutes or more over a 24-hour period starting from 2 days before illness onset (or, for those without symptoms, 2 days prior to test specimen collection) until the time the patient is isolated.

Isolation (for cases of COVID-19)

Confirmed and probable cases of COVID-19 should isolate at home away from others for at least 5 days from symptom onset or 5 days from date of test if no symptoms. Cases may return to the workplace on or after day 6 if they have been fever-free for 24 hours without fever-reducing medication and symptoms improving. No negative test is required to return. Cases must mask through day 10.
Suspect cases of COVID-19 (people who have tested due to symptoms) should isolate at home while awaiting test results.
https://www.cdc.gov/coronavirus/2019-ncov/your-health/isolation.html

After Being Exposed to COVID-19

Close contacts to COVID-19 do NOT need to quarantine but should begin wearing a mask as soon as they find out they were exposed. Close contacts should mask for 10 days from date of last exposure to the case. Masking is for both vaccinated and unvaccinated contacts. Contacts should watch for symptoms, and if they develop symptoms, they should stay home and get tested. They should stay home until they know their results.
Contacts should test on day 6, even without symptoms. If negative, continue masking through day 10. If positive, isolate away from others. If a contact has recovered from COVID within the last 30 days, there is no need to test.
https://www.cdc.gov/coronavirus/2019-ncov/your-health/if-you-were-exposed.html

Symptoms of COVID-19

People with COVID-19 have had a wide range of symptoms reported – ranging from mild symptoms to severe illness. Symptoms may appear 2-10 days after exposure to the virus. Anyone can have mild to severe symptoms. People with these symptoms may have COVID-19: fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, diarrhea.

The following is the CDC's updated guidance for COVID isolation and quarantine:

(Revised  Aug. 12)