“What do you think? If a man has a hundred sheep, and one of them goes astray, does he not leave the ninety-nine and go to the mountains to seek the one that is straying? And if he should find it, assuredly, I say to you, he rejoices more over that sheep than over the ninety-nine that did not go astray. Even so it is not the will of your Father who is in heaven that one of these little ones should perish.” (Matthew 18:12-14)

This is a similar parable to the parable of the Lost Sheep recorded in (Luke 15:3-7).  Both Matthew and Luke use this brief illustration of a flock of a hundred sheep with one missing whom the master of the sheep seeks out and rejoices over, but the contexts are different.  My opinion, Luke describes from an evangelism perspective while Matthew describes from a pastoral perspective. 

In Luke, we find tax collectors and sinners described as lost sheep. The Pharisees excluded and refused to associate with tax collectors and sinners. But Jesus cared about everyone. There is great joy in heaven over each sinner who repents.

While in Matthew, we find that people who struggle with obedience and stumble into sin are described as wandering sheep / sheep that go astray. The sheep has not merely wandered off but has actively been deceived or misled to be away from the remainder of the ninety-nine sheep. Jesus also cares for them.

Just as a shepherd is concerned enough about one lost sheep to go search the hills for it, so God is also concerned about every human being He has created including children. God is longsuffering and He does not want anyone to perish but all should come to repentance (2 Peter 3:9). 

Like the good shepherd, we should care, seek out and nurture both the lost and wandering sheep.