Simply put, the "call" is God's invitation. Nothing more. Nothing less. I personally believe that every person experiences two kinds of calling in their life.
The first is the general call. This is the invitation from God to join Him in a relationship. Man's relationship with God was broken by the original sin of Adam and Eve. Every person ever since that day has been born with a sinful nature that rejects God and misses His purpose for their life. And, every person since that day has been offered an invitation to re-join God in this relationship that He desires to have. How we respond to that invitation or call is up to us. It is the free will that God gave us and it is the difference between becoming a Christ follower and living a life completely separate from God.
The second call is the specific call. I believe this has been the concern of my recent conversations...how do I know if God is calling me to do something specific? Let me assure you that He is. If you have responded positively to the general call God has given you, then your next step is to find his specific call or purpose. Some are called to ministry full-time. Most are not. And answering your specific call is not dependent on what you do but the attitude of worship with which you do it. In other words, you don't have to be a pastor or missionary to have a call. God calls some to be teachers, lawyers, firemen, housewives, financial planners or trash collectors. Your obedience in serving out God's call is determined by the heart with which you serve.
I tell my RCC family frequently that we were created for two purposes in our life: to enjoy His grace (general call) and to extend His glory (specific call). God smiles when people choose to receive His grace. He also dances with delight when you and I turn to extend His glory so that others may know. If you've never answered either of these calls in your life, I encourage you to pray, talk with a minister/counselor/friend and determine to rearrange your life around these two purposes. You'll find you spend less time chasing meaningless things. You'll also find greater satisfaction with who you are in Christ when you understand that you were created to be uniquely you...all for His glory.