Introduction to Philippians
I love Philippians. This is probably Paul's most informal letter and requires the least background to understand what he's writing here. Whereas a letter like 2 Corinthians or Galatians has a lot of history behind it that is important to get if you're going to rightly understand the letter, Philippians is pretty understandable right away. Because of that, I'm going to skip the usual introductory stuff and jump right in.
Most important words to notice: joy and suffering. Paul is writing this letter from prison, but you'll see joy-related words (such as rejoice) again and again, as well as a lot of references to suffering.
Also, the introduction of this letter is particularly important:
"Introductions play a huge role in communication. In some Asian countries there is a formal protocol for exchanging business cards in introductions. In diplomacy, ambassadors present credentials to the host country from the nation they represent. In academic circles, the curriculum vitae is a formal résumé highlighting the scholar’s significant achievements and qualifications. Each of these approaches accomplishes the same task: giving us the most important information we need to know about the person for the particular context. The fact that a person is the “Mid-Atlantic ballroom dancing champion” might be interesting, but would not be relevant in a diplomatic or academic setting...
...Likewise, Paul chooses slave as his credential because the qualities of a faithful servant mesh with his exhortations of the Philippians. He has willingly accepted his circumstances (imprisonment). He views them as ordained by God for the advancement of the gospel and exhorts the Philippians to adopt this perspective. Far from being a victim, Paul rejoices and will continue to rejoice in these circumstances. He’ll unpack the key to this throughout the letter. But fundamentally it begins with a willing, thankful submission to God. The slave credential sets the stage for the exhortations and affirmations that follow." 
In short, Paul introduces himself this way as if to say "I belong to Jesus and will do whatever He wants me to do." This relates directly to what Paul goes on to say in verse 6. It's unusual, because Paul doesn't just say "I'm thankful that God is my master and is working in us all." He doesn't use the term "God". Why?
"...instead of calling Him God, Paul uses He who began a good work in you. The change from the standard God or Lord is risky because his audience has to figure out to whom Paul is referring. Paul is forcing us to think about God in a specific way—in this specific context. Paul has a reason for doing so.
Think about all of the different qualities we might conjure up when God is mentioned. By using an expression other than the normal one, Paul forces us to think about the particular quality he highlights.
Despite the negative appearance of the circumstances, God is still in control and still accomplishing His purposes in the life of Paul and in our own lives. Beginning the “good work” was not a mistake that will be left incomplete. Adopting God’s perspective on the situation requires us to give up our wrong perspectives. Paul affirms in 1:7 that this is the proper way to think about things, implying that the Philippians should think this way." 
Lots of good thoughts in this short section today, so we can't possibly cover it all, but I think this covers a lot of the main points that Paul is trying to get across: take joy that God is your master, is in control, and continues to work in us all!