In this context, “professional contacts” means both everyone you have worked with but all those people with whom you have interacted well: people who have interviewed you with for jobs you did not get, if the interview went well, are examples of those people in the latter category. Together they make up the group of people who may over time provide you with a lead to a job, a contact at someplace where you might like to work, or a lead to someone you might want to hire, including them.
As your career progresses and you have worked in more organizations with more people, your professional contacts will become one of your most important professional assets, both for advancing your own career and for helping others’ careers. You should treat them with the care valuable assets deserve. Like your résumé, your contacts should always be up to date.
Decide where you will keep them. I have seen people use everything from a stack of file cards with a rubber bands on them to Outlook to Google contacts. Wherever you decide they belong, use that location as the master and make copies from it. When you get new information, update the master, and from there either electronically update other sources or manually copy information.
Do regular backups. Follow the rule of three: keep one local back up, one cloud backup, and one backup to a(nother device). Some kind of backup is particularly important if your master is in a hard-copy system like file cards or an address book. I recommend doing backups no less than once a month, particularly if you are diligent about adding new professional contacts. Cloud systems like Google contacts that auto-deploy to your Android phone, for example, are handy and helpful, but don’t reply on them alone for storing your contacts.
LinkedIn connections suffice for purely professional contacts. Set a calendar reminder on your desktop, cell phone, or paper calendar to add two new people to LinkedIn every two weeks. Get in the habit of adding people to LinkedIn whenever you have confidence that they are people you would either be pleased to introduce to someone you respect or would be pleased to do them a favor. I recommend not adding anyone who contacts you randomly, including people who describe themselves as recruiters. For one thing, anyone can call him- or herself a recruiter; for another, if you use LinkedIn to provide introductions to two people who do not know each other but know you, you want to be sure you are introducing people you know professionally: it’s your reputation at stake here, too.
Women in workplaces with few women. Women in workplaces with few women do well to make sure they know every other woman in their workplace at every level and in every role, and to have their work contact information, even if they do not work with them directly. Women can become isolated and discouraged in those workplaces, and it is important for them to check with other women about workplace behavior they find odd, unpleasant, or worse. “Every role” means every role – if you are in the managerial or professional workforce, make sure you are pleasant to and chat with cleaners, receptionists, security guards, and janitorial staff. First, everyone deserves professional courtesy; second, you never ever know who you will work for or who will work for you, or refer you to your next great employee or job; third, if something is wrong at your workplace, you may hear about it from them first, as people in those jobs often interact with a wider range of employees than you may.