Okay. Here goes nothing. I'm going to be thorough in the name of clarity, forgive me if I say the obvious.
Strikeouts: Pitchers have great control over these. You should generally trust pitchers k/9 rates. If they threw 8 K/9 in 220 innings last year, it's not too likely that they are secretly a 9 k/9 pitcher that's just been unlucky. If they threw 8 k/9 over a 200 innings you have to trust that it's a fair reflection of their ability. Likewise, if they were 8 k/9 last year, but are 6.5 k/9 this year over 200, you probably want to assume that this reflects a real drop in their Stuff rating, especially if they're older. It's too big a drop to be a fluke. Stuff goes down the older a pitcher gets, generally.
Walks: Pitchers have a lot of control over these, but not as much as strikeouts. Everything I just said above applies to BB/9 and control, except that control is often the last attribute to peak for pitchers, and is often the last one to drop.
Home Runs / 9: This is a combination of Movement and Groundball rate. Movement means that balls are less likely to be hit for home runs, groundball rate makes it more likely that the balls never have a chance to be home runs. Home Runs / 9 are hugely volatile; a swing of 0.5 HR/9 from 200 ip season to 200 ip season is not unreasonable. For this reason HR / 9, while a very powerful stat (because home runs are baaad to give up), are not very strongly tied to pitcher performance. I mean, there's a serious skill there, but it's not as consistent.
Everything Else: Within reason, is not the pitcher. Hits allowed once put into play are only a bitty bitty bit the pitcher, and the rest is mostly batter skill / fielding skill.
What this means is that if you see a pitcher give up five runs in four innings and his stat-line reads: 8 hits allowed, 8 strikeouts, 1 walk and 0 home runs, then your pitcher pitched his ass off and just got unlucky. Because 2 strikeouts per inning is awesome, as is 1 walk in four innings. The bad part is the 8 hits and, again, is not the pitcher.
So. What composite stats are there to decode this?
FIP: FIP stands for Fielding Independent Pitching. It's basically an ERA made with only HRA, K and BB. So it's kind of like what the pitcher's ERA would be if the defense were completely average.
FIP- is a stat that compares the pitchers FIP to the league average. A FIP- of 100 is league average. A FIP- of 120 is 20% higher than league average. A FIP- of 80 is 20% lower than league average.
WAR: WAR is wins above replacement. Basically, WAR takes your pitchers FIP, compares it to replacement level (ie, what the formula thinks you could get just by grabbing someone from AAA or a scrub off free agency, an easy replacement) and figures out how many wins your pitcher has been worth to you compared to someone who would cost you nothing.
Recap: WAR is
the stat for starting pitchers; it combines quality (FIP) with quantity (IP). Obviously it can be a little misleading at times. If a career 0.6 HR/9 pitcher is allowing 1.2 HR/9, his WAR will get worse, but it's not a good indicator of how good he is, only of how good he has been. Likewise, I don't think OOTP's WAR adjusts for park effects, so if a Colorado pitcher and an Oakland pitcher have the same WAR, the Colorado pitcher is probably better because home runs are much easier to hit in Colorado than Oakland.
With relievers, understand that they only pitch 50-100 innings, so you won't get good sample sizes out of them. You could have a monster reliever still allow 1.5 HR/9 over 70 innings as a giant fluke, and it could still have nothing to do with them. So be patient with their stats, and trust their K/9 and BB/9 much, much more than their HR/9.
I confess, I'm a serious SABR guy. So take the following with a grain of salt. I have no idea what any of my pitchers' W-L record is, or what their ERA is. Those things are good at telling you what happened, but not very good at telling you
why it happened. Because the entire idea of talent evaluation is to figure out how well someone played, not how good the result was. Compare a pitcher with a 7IP, 5 ER, 13 H, 10K, 1 BB and 0 HRA to one with 9 IP, 0 ER, 2 H, 5 K, 3 BB and 0 HRA. One pitcher had an ERA of zero, the other had an ERA of 6.42. But the first pitcher is likely the better pitcher, perhaps much better. The first pitcher had a FIP of 0.77, the second pitcher had a FIP of 3.76. And their WARs would reflect it.
Anyhow. I don't know if that's too much detail or not enough. Don't hesitate to ask follow-up questions