tina asked: "What makes a weak hadith and what makes a strong hadith?"
The "strength" or "weakness" of any hadith refers to the isnad or chain of transmitters of the hadith. Each hadith has an isnad that tells us who related the hadith from person to person. Each of these persons has had their life reviewed, especially as it pertains to the transmission of ahadith (the plural of hadith). For example, was that person trustworthy, how good was their memory, were they known for having created any spurious ahadith or falsifying in any way any hadith, and so on. The grading of all of the transmitters in each isnad would determine the status of the hadith, whether it is sahih (strong), hasan (fair), daif (weak), munkar (denounced), or maudu (forged).
You might find this article, The Science of Hadith of interest.