If you are unable to repay your debts, your creditors may employ a debt collection company. It is the job of the debt collection agency to contact you, explain to you the terms of your repayment (now with their involvement), and arrange for payment. If you are in financial trouble, you should bear in mind that your creditors would rather work out a new repayment schedule, possibly with lower monthly payments and/or a temporarily lower interest rate, considering that they would have to pay the debt collection company a fee or percentage of the amount owed to do it for them.
If your lenders do end up going with a debt collection agency, usually the first step taken by the debt collection company is to write you a letter, informing you of their involvement in and actual management of your account. If this happens, it is in your best interest to make sure the debt collection company and its letter are legitimate and then make repayment. If you do not, you will probably be issued a Final Demand, informing you of future legal action and possible bankruptcy. This demand is usually accompanied by phone calls. If you do not respond to this debt collection effort by making an arrangement with them, legal action will usually be taken.
Note that debt collection will have a very negative impact on your credit history. Your creditors will have made it known to the credit reporting agencies that you defaulted on your payments, and the debt collection agency will be sure to make a note of their involvement as well. Having these things on your record will make it difficult for you to secure credit in the future, and if you're able to get a loan or a credit card, your interest rate will be much higher than average due to the high amount of risk associated with lending to you.
The best thing to do is to be proactive about your debt, and if you feel yourself becoming overwhelmed, speak with your creditors about your situation to see if they'll be flexible with your payments or with your interest rate until you get your finances back on track. If your creditors are unwilling to assist you, consider a debt management or a debt counselling service. They may be able to get your creditors to reconsider your repayment terms.