Lag and connection problems

Hi, I came here wondering about lag. What causes lag? The speed of the Internet, the latency, or the distance between 2 or more players. I have a 4mbps internet and people tend to complain that I lag but it's fine for other people. I have my router wired. If it is because I have low speed internet, what is the ideal speed for playing for example COD or BF on? 2. How do I enable my internet to be able to substan 2 or more people playing because I lag when somebody is watching a vid on YT or downloading some files. Pls help