"Donkey" and "fish" are terms used in poker to describe bad players. Most people use them interchangeably, but there is a subtle difference between them. Fish are generally players that are fairly new to the game of poker. They play weak starting hards, make bad calls, rarely raise, and don't fold when they should, simply because they are novices and don't know any better. You can usually find plenty of them at low-limit or micro-limit games. Donkeys, on the other hand, are more experienced players. They have probably been playing for a while, know a bit about the strategies of playing poker, and may have even read a book or two about poker. However, they continue to play weak hands and make mathematically bad calls, probably out of habit or just because they like to gamble and play a lot of hands. These players can be found at higher limit games.
It should be obvious that good players like to play against bad players. Over time, the mistakes that bad players make will result in them losing money, and good players who are able to take advantage of these mistakes will, over time, make a profit. So, when you look at the big picture like this, you gotta love donkeys and fish - these are the people who are going to be paying you off.
However, poker is also a game of luck, and even though you may be a more skillful player, sometimes you will still lose to a bad player simply because they got lucky. These are the times when you just hate donkeys. It can be frustrating to lose to someone who made a questionable call and got lucky with the cards. It can be very frustrating when this happens, and they take you for your entire stack. And then there are the times, like four nights ago, when it can be 
extremely frustrating 'cause it happens twice in one night!
That night, I ended up losing $376, mainly because of two hands where my donkey opponent got pretty lucky with the cards. The first hand, I'm in SB with 
77. UTG+1 calls for $2, BTN calls, I raise it to $8. At this point, I'd be happy to just take the pot right now. With just two calls ahead of me, I'm probably ahead with my pocket 7's, but I'd rather not see the flop if possible. However, both opponents call. Flop comes up 
3K2. Since I was the aggressor, I lead out with a bet of $15 into a $26 pot, hoping to take the pot right there. If someone calls, I'll probably have to fold on the turn. Both opponents call, and the turn brings a 
7. I check, knowing someone behind me will likely bet, and I can check-raise. UTG+1 bets $20 into $71 pot, BTN calls, I raise it to $84. UTG+1 re-raises all-in to $103, BTN calls, I call. River is a 
6, I go all-in for $73, BTN calls. UTG+1 flips over 
AKo for top-pair-top-kicker, BTN flips over 
45o for a straight! I lost $199 in that hand because the donkey called a 4x pre-flop raise with a crappy 
45o, then called a huge re-raise on the turn with his open-ended straight draw, when he wasn't getting the proper pot odds to call (pot was laying him only 3.35:1 odds, while the odds of making his straight draw were 4.75:1).
The other hand, I'm in SB with 
AA. UTG, UTG+1, MP, and BTN all call for $2. I raise it up to $11 to try to narrow down the field. BB calls, and then the rest all proceed to call! Right away, I knew I could be in trouble. 
AA is a great hand versus one or two opponents, but versus five the odds go way down. Flop comes up 
24J rainbow. This is a fairly harmless flop for me, so I bet out $35 into a $66 pot. BB calls, folds to BTN who also calls. Turn is a 
6, I push all-in for $88 into a $171 pot, BB raises all-in to $158, BTN calls for $111. River is a 
9. BB turns over 
KK, BTN turns over 
J6s for two pairs. I lost $134 in that hand, because the donkey player called a big pre-flop raise with a crappy 
J6s and managed to hit two pairs.
Anyway, after that night it continued to go downhill. I played again two nights ago, and lost another $180. My 
QQ vs 
AJ, opponent rivers a third jack. 
AA vs 
55, opponent flops a set. By then, my bankroll was down to about $900. Still good considering I started with $50, but sucks considering it was over $1400 a few nights before.
Well, I played again tonight, and I'm glad to report that I'm once again loving the donkeys. I made $505 this session, mainly due to three hands.
First one, I'm in MP with 
KsKh. UTG and UTG+1 call for $2, I raise it to $8, MP+2, BB, UTG, and UTG+1 all call. Flop comes up 
49T with 2 clubs. It's checked to UTG+1, who bets $11 into $41 pot. I raise it to $30, MP+2 re-raises to $60, folds to UTG+1 who is all-in for $26, I call. Turn is a 
5. At this point, I don't think he has 
AA, as he didn't re-raise me pre-flop. I also don't think he has a set, as he probably would have pushed all-in after my raise on the flop. However, I decide to check and call to play it more conservatively. So I check, he bets $100 into $187 pot, and I call. River is another 
5. I check, he bets $100 into $387 pot, and I call. He turns over 
AcJc for the missed flush draw, UTG+1 turns over 
8h7h for the missed open-ended straight draw, and I win a whopping $316 in this one hand.
Second hand, I'm in BB with 
AA. UTG calls for $2, MP+1 raises to $7, CO and SB call, I re-raise to $32, it's folded to SB who calls. Flop comes up 
994 with 2 clubs. SB bets $11 into $80 pot, I raise to $30, he calls. Turn is a 
3. He bets $45 into $140 pot, I raise all-in to $145, he thinks for a while and folds, and I net $120.
Third hand, I'm in MP with 
AhJc, UTG+1 calls, I call, BTN and SB call, BB checks. Flop comes up 
5J7 with 2 clubs. SB bets $2 into $10 pot, UTG+1 calls, I raise to $12, only SB calls. Turn is a 
T. SB bets $10 into $36 pot, I raise it to $30, he calls. River is a 
4. SB checks, I bet $50 into a $96 pot, SB calls. He turns up 
9d7d for third pair and a missed gut-shot straight draw, and I net $99. I asked him afterwards why he called my river bet, and he replied that he made a pair of 7's - go figure!
Anyway, I'm back to just a bit below where I was before I had that huge downswing, so I'm pretty happy again. Variance can suck, but it's an important part of playing poker. That's why you need a sufficient bankroll to play at the limits you usually play. For NL games, the general recommendation is that you need between 10-20 buy-ins at the level you play. That's also why this challenge is so risky. If I had been playing $500 NL like I was supposed to, with just under three buy-ins in my bankroll, I probably would have come close to busting out from that 2.5xbuy-in downswing I had.
Earnings-to-date: $4197Challenge bankroll: $1395