I noticed some players with very few wins but their level is about 25. How is this possible? Do they play with bots? Or do they really almost never win? Or do they play incredibly well?
Lets look at what is required to get to level 25+
The received Experience Points are calculated at the end of each game; wins are worth more than losses, and the longer the game lasts, the more XP you gain.
Assuming an average XP gain to be about 110 XP per normal game, one can reach summoner level 25 within 246 normal games. Using XP boosts will double the amount of XP you recieve. So, lets assume someone got the maximum XP which for winning a normal game will be around 200 + the use of XP boosts = 400 xp per win. One can get to 25 in 68 games. Source
However, it is important to note that depending on where you are looking will depend on what information is shown to you. For example, if you are seeing the 10 wins from a summoner at the end of a normal game then it will only show the stats from winning normals. This means it won't show any stats for winning any other mode (Dominion, ARAM, CO-OP vs AI, custom games).
It is quite possible that someone could reach 25 through playing only Dominion, ARAM etc games and has only played around 20 games of normal. For you to get a real idea of how many games someone has played I would recommend using lolking.net. It should show all the games someone has played in which mode, their wins, losses and many other statistics.
There are basically two ways to achieve this:
The first would be by just playing other Queues than normal 5v5. Other Game queues aren't listed as "wins" and thus the low amount of won games in normals.
The second method is rather weird and requires him to really be losing all the time. This would require a incredible high amount of games for him to lose which is highly unlikely to happen.
Either way there is no "hacking" or "exploiting" involved.
Based on the Wiki :
There is both a minimum and a maximum amount of XP and IP a player can get from a game. Current (July 2011) formula for XP winning a Normal game at level 15-19 is roughly 3m+25, where m is the number of minutes the game lasts, capped at 198. Intermediate Bot games at level 10-14 use the same formula. At level 15-19, Intermediate Bot games subtract a 25% penalty from that formula, and give XP for only the first 180 minutes a day of play.
Also, from the table on that article, Level 25 can be reached from 27020 Cumulative EXP.
Even with an XP Boost and the cap, it will take around 60+ games before you reach the needed XP.
Maybe you're only looking at the Summoner's Rift stats. Try looking at his stats on other games ( Twisted Treeline, Dominion )