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First of all TTL means Transistor Transistor Logic and it just indicates the technology which is being used in the IC. Secondly 74LS90 is a Decade counter which means that i can count from 0 to 9(looks like BCD?).
The above link gives the timing daigram of a decade counter.The last output bit(Qd) of the first 7490 is given to the clock i/p(CKA) of the second. When the 1st 7490 goes from 9 to 0, the next 7490 increases by 1 and every time this happens the second 7490 increaes by 1.Here is a link(there is some thing wrong with my windows otherwise would have edited that for you)