A single crystal  often consists of a distribution of regions which are highly oriented.  The polycrystalline material consists of single crystals described above but of very small sizes of the order of micrometers but randomly distributed when there is no preferred orientation. The scattering of X-rays by atoms is the point to be considered in that case. When there is periodic arrangement of atoms the X-rays (single crystals) will be scattered only in certain directions when they hit the formed lattice planes ( formed by atoms). This will cause high intensity peaks ( the width of the peaks is small for single crystals but also depends on other variables).  For polycrystalline materials X-rays will be scattered in many directions leading to a large bump distributed in a wide range (2 Theta) instead of high intensity narrower peaks.

A single crystal  often consists of a distribution of regions which are highly oriented.  The polycrystalline material consists of single crystals described above but of very small sizes of the order of micrometers but randomly distributed when there is no preferred orientation. The scattering of X-rays by atoms is the point to be considered in that case. When there is periodic arrangement of atoms the X-rays (single crystals) will be scattered only in certain directions when they hit the formed lattice planes ( formed by atoms). This will cause high intensity peaks ( the width of the peaks is small for single crystals but also depends on other variables).  For polycrystalline materials X-rays will be scattered in many directions leading to a large bump distributed in a wide range (2 Theta) instead of high intensity narrower peaks.

Do you see spots or rings (or spotty rings)?

SEM shows the apparent observation of your sample morphology. However, there would be secondary grain boundaries inside one grain in SEM photos. The secondary grain is called crystallite, which is deemed to be a perfect crystall with only one direction of crystall lattice.

The answer given by Maria Vasilopolou explain the difference between the two cases by using XRD

Also if you have the materials surface image particularly from HRTEM, you can easily distinguish between if the material is polycrystalline or single since for single crystals all surface atoms are systematically aligned while for polycrystalline all ingots (small crystals) are distinguished      

if you have a 2D detector - single crystal would give you spots/elongated spots; powder would give you rings

Manjeet! Depends on what size your coherently diffracting domains (grains, crystallites, particle) are and what their relative misorientations are? If the particle you examine satisfies the Bragg condition over a narrow angular range then you may conclude that as monocrystalline, for instance. You'd really have to define what exactly you mean when you scribe "single crystals and polycrystalline". You'd have to define this based on misorientation and coherence.

Lots of good suggestions already from the experts in this matter. Thanks!

The correct universal answer to your "basic" question is real time in situ XRD Topography or Bragg XRD Microscopy. You may use the antiquated photographic film for this purpose or join the modern world and use a high spatial resolution (29 um) 2D real time detector with high dynamic range (16 Bit). There is no need to go to an esoteric cost prohibitive technique like electron microscopy yet. Additionally, there is no need for sample destruction or sample preparation yet. All measurements may be made in situ with XRD. Attachments are available for nearly all existing diffractometers to allow such measurements using

Thanks to all experts for good suggestions n extensive answer regarding my query...

Dr. Lawrence Margulies.... SAED pattern that appears composed of  rings for one sample and spoty rings for other sample.....What does it mean?

Three examples of the Debye-Scherrer rings of progressively larger "particle" sizes for : ("spotty to continuous" as Volker and Larry suggested)

Another example of effect of "particle size" on the diffraction pattern in transmission Laue mode: https://www.flickr.com/photos/85210325@N04/7920887956/in/set-72157632728981912

With higher intensity as in the synchrotron each coherently diffracting domain may be resolved and imaged. The advantage with the XRD methods compared with TEM is the in situ nature of the observations without sample destruction or sample preparation. TEM has orders of magnitude higher spatial resolution at present, temporarily :-)

Technological advances are bridging this gap rapidly!

How to distinguish between a single crystals and poly crystalline materials using XRD?

This is my interest! Since we are in the presence such erudite company and we do have an objectionista among us, I'd like to delve further into this XRD aspect. Admittedly, my expertise in TEM is at best basic. I'll leave TEM its pros and cons to you experts. I'm still learning :-)

I intend to add a lot more on XRD, real time stuff! Check out this video from a ZnSe wafer that possible contains mono crystalline, poly crystalline, pseudo amorphous, voids, sub grains, etc. Rich data set collected with a Panalytical HRXRD system and a Panalytical PIXcel area detector by the good folks from Panalytical. We just analyzed and displayed the data and analyses results. You judge! We need to define some terms, I'd say!

Terms like mono crystalline, poly crystalline, grain boundary, sub grain boundary, coherently diffracting domain. What amounts of miss-orientation will distinguish one "grain" from another? When does the sub grain turn into a grain? How much miss-orientation?

Compared to other low defect materials like GaAs, Si, GaSb etc., the ZnSe wafer was relatively of lower quality. Both the Bragg peak shift and FWHM were significantly higher. However, the diversity of Nano structure present in the ZnSe wafer are to say the least, rich & instructive!

https://www.youtube.com/watch?v=dFCQS8oUyT0&index=37&list=PL7032E2DAF1F3941F

Which are the best scientific research paper editing softwares?

A pen.

lawe photograph for single crystal will give XRD point with their arrangements representing crystal planes while for polycrystalline each of thr microcrystals forming the material will the same picture produced by the single crystal and then their collection will produce lines each of them represent a plan